Look. My character is the perfect juggling basketball because he literally has zero move that hits downward. Combine that with his 2nd slowest airspeed AND floatiness.
Nair is a sex kick that surrounds Luigi with a hitbox and comes out a frame 3. With customs, you also have Burying Headbutt. Along with Dair, that's 3 moves that can hit downwards. There is no character in this game without a move that hits downwards, but there are few characters with moves that hit in more than one direction much less surrounds their hurtbox with a hitbox and very quickly like Luigi's Nair.
Rosalina is also a floaty with average, but not slow air speed. Her floatiness allows her to escape combos since most of the cast don't have Falco, Jump Art Shulk, Greninja, or ZSS's high jumps. Or take Kirby who also has slow air speed and is floaty. Same dealio.
Luigi runs barely faster than Falco (The difference is almost negligible). He can crawl, so what? Ain't gonna do a squat other than Fox MU. His floatiness isn't going to help him because he gets juggled hella back. Oh, and also, Falco has better mobility because aerial specs + Phantasm.
Luigi, Ike, Samus, Wario, Kirby, Mii Swordfighter, and Lucas are just right below average speed of Mario and Ryu. Falco, Olimar, and more are just right under with Falco and Kirby's being more notable because unlike Luigi, Olimar, Samus, and Wario, they lack any means to safely approach or force approaches. High jumps, multi-jumps, and good walking speeds - Falco's that is - don't do much when it comes to raw horizontal movement.
Crawling at least gives Luigi another movement option alongside basic movement options like running, walking, foxtrotting, dash dancing, perfect pivots, short hops, full hops, and burst movements through Luigi Missile (customs) and Luigi Cyclone. How effective it is depends on the character, but having an option is good, especially when few characters have that option.
Falco's aerial specs are fast fall speed which bites him back as all fast faller feel unless they have insane air speed or a quick aerial to throw out, poor air speed, and a high jump. That's good vertical mobility, but poor horizontal mobility. For ground options, he only has a fast walk speed, a below average run speed, and Falco Phantasm's butchered hitbox makes it less safe than Fox Illusion or even the random Luigi Cyclone. This is all compounded by the fact Falco's approach is *** compared to the entire cast. He can't tank things like Ganondorf and Bowser, he can't camp you like Sonic or Sheik, he can't do hit and runs like Fox, he can't cover his approach like Luigi and Yoshi, he can't cover retreating force approaches like Mario and Mega Man, he can't control the stage like Duck Hunt or Pac-Man, he can't throw out safe moves like Sheik, he can't even snipe off huge rewards like Zelda, and he definitely can't bum rush like Captain Falcon or Meta Knight. Bait his Reflector if you have projectiles, camp him out, and force him to approach or punish his approaches and Falco is one dead, pathetic pheasant.
Luigi struggles to get in on possibly half of his MUs (Or at least, annoying). If he doesn't have the whole 'I get lotsa damage when I get in', then he'd be just a worse Mario clone.
That's better that struggling with almost every MU. You don't know pain until you've had Zelda force approaches and effectively punish because Falco's too slow to bypass Nayru's Love. Luigi has a usable projectile like Toon Link, Link, Fox, Rosalina, Mega Man, Sheik, Diddy, Samus, Mewtwo, Lucario, Ness, Lucas, and more. That alone lets him play at a safe distance and at distances of his choosing. Falco only has two: up close or at the character select screen.
Most of the cast does the "I get lotsa damage when I get in". Fox, Falco, Peach, Sheik, Meta Knight, Ganondorf, Roy, Marth, Lucina, Pit, Palutena, Diddy, Donkey Kong, Robin, Captain Falcon, Yoshi, Ryu, Greninja, Ness, Lucas, etc. There's few characters that do a ton of damage when they're away. Duck Hunt, Snake, and Brawl Falco come to mind as those few. Even Rosalina can't since she puts Luma there or Olimar who doesn't exactly have mega range.
Comparing him to Falco is, tbh and imo, silly. Both characters play very differently.
I was giving an example of a character whose attributes hurt him. Do you want me to talk about Ike, Ganondorf, Bowser, or Fox?
Oh, what's that? You want to nerf his damage output? Then give me this medicine that boost my mobility to compensate for that. Hell, if you nerf his damage output it'd straight up ruins his entire combos because he relies on hitstun, which is determined by damage.
Did I say I want Luigi to only do 1% out of grab and throw? They are few, but there are characters who aren't mobile, but still can kill well and reliably and rack up damage very quickly. Ness and Lucas come to mind along with Luigi.
The main point is how to keep Luigi's damage output and throw game, but toning it down and hell, we might not need to tone those down as my other patch speculah post suggested. Sometimes it might be better to leave one character alone and strengthen others. The main complaint about Luigi (and Sheik) is how low risk and high reward they are. Luigi getting everything off of D-throw is what irks people while it was the absurd power of Uair and D-throw setting it up that annoyed people with Diddy. That's a reason why you have people calling Luigi "braindead" because he gets combos, juggles, spike setups, jab locks, and kill confirms off of one throw.
People suggest making the end lag on grab more so Luigi can get punished for making mistakes or the end lag on Fireball, but I'm a bit against that since end lag kills as evident with Falco and his 58 frames - 49 if you use it in the air - of end lag on Blaster. Perhaps lower clank stun instead to allow people to time a "parry". One thing that I thought of was making Luigi's other throws be able to combo and make his D-throw only be able to combo into less options while still being able to have strong options. This could turn Luigi into a grappling trickster and force the opponent to guess which throw and DI'ing incorrectly could let Luigi punish hard. So, U-throw still kills, but allows Luigi to Uair, Nair, Luigi Cyclone at low to mid-percents, and even Super Jump Punch if he reads it right, F-throw is open to Fair, RAR Bair, and run under Up Smash, D-throw is open to Nair, Uair, Dair, Luigi Cyclone, and Super Jump Punch reads. It's giving Luigi more options, but making it so Luigi has to plan and pick his options.
After all, other than his combos, what else he has? Fastest attack speed and good damage output? Combined by his bottom 5 mobility? No. I ain't gonna punching a thing with that.
A lot of other characters also have fast attacks speed and good damage output. Mario, Sheik, Fox, Captain Falcon, Meta Knight, Yoshi, Falco, Peach, Sonic, etc. It's certain things like if and what are their hit confirms, how their moves function, how they move and flow, and what goes against them that defines them.
In terms of bottom 5 mobility, Ganondorf, Robin, Dr. Mario, Triple D, and Zelda already have that covered.