I could respond to all your points but it seems it might be like talking a religious person out of their religion.
Another Marth's Witness reporting for duty.
So I'll just prove one of your points terribly wrong again.
If Marth had Roy's D-Tilt:
D-tilt > Tippered F-smash
= Easy KOs starting at 50-60% for Marth
This is somewhat true, but it would be DI dependent. The biggest issue with it is that Marth would lose his main spacing tool and would have to be closer than what is comfortable in order to land this d-tilt. It would also be way less effective for edgeguarding. This move would be preferred by pre-2013 Marths where it was more about the hard punish game, but modern marths are more interested in the exceedingly safe spacing tool of d-tilt and not being commital. Both would work fantastically with Marth, but it's definitely not true that "any Marth" would prefer Roy's d-tilt. Plus, I think you can smash DI this move down to negate getting popped up entirely, but I'm not sure on that.
And Marth's counter is garbage for ledge-guarding and in general it's not even worth it in lieu of other moves (7% always WOAH!). The tiny knockback just keeps them coming right back to the ledge.
If Marth lands the counter on a fire-spacie (Which is very easy because of the huge amount of active frames, which covers both direct & wall-riding) then the spacie is guaranteed to lose the stock. The spacie isn't outright KO'd by the counter, but the counter sends them down-and-away positionally after hitstun, which guarantees Marth's fair or bair to KO them very safely. Marth's counter is AMAZING for edgeguarding because of what it sets up, not because of its own KO potential.
Roy's counter can end a stock early in one blow for Falco or Fox (their Up-B's knockback and damage x 1.5 is huge)
It is a solid edgeguarding tool, but it's way harder to both land & cover multiple options with, given its being active on frame 11 and not being active much longer. Likewise, since Roy does
not have the option to drop down and fair/bair like Marth does to cleanly end a stock after a counter, the spacie actually does get a chance to come back at lower and mid percents. Marth's counter can guarantee a stock loss if it lands at 15% -- Roy's counter can not, and you have to try to land it again (which, again, is not as easy due to it's slow startup and low active frames.) Roy's is definitely
flashier, but Marth's is definitely
better due to being a guarantee
and easier to set-up.
and it keeps opponents from throwing out really hard reads because they know that a falcon punch or a charged donkey punch can mean OHKO right back at them (it's pretty easy to intercept a Falcon Knee, you pretty much know when it's coming in the air). Not many current Roys use counter much (only on edge and seldom) because they aren't skilled enough or don't have fast enough reactions it seems.
This is not realistic. Roy is of the weight and fall-speed so that if you miss a single counter, you have lost a stock as no one decent with a decent character should drop a punish on a Roy, be it a chain-grab, a gimp, or a juggle. The reason this is a problem is because human reaction time is going to be in the line of around 15 frames with all-things-considered. This means that Roy can
not counter moves on reaction and hope they hit, unless they are moves like Bowser's f-smash. Falcon knee is active on frame 18, so Roy would have to both input the counter by frame 7, which means both reacting perceptively AND physically within just 1/10 of a second, which I hate to say isn't possible when considering the highly malleable nature of smash options. If a knee is so telegraphed, then the Falcon is bad. If the Falcon actually tries to Falcon punch, then the Falcon is bad. Also, I think the Donkey Punch is even faster than the Knee, but idk for sure.
Unfortunately there just aren't any good Roy mains at all for Melee, just like there weren't any really good Yoshi mains until AmSa came along.
This is not a realistic comparison. Yoshi has been known for a long time to have really good options, but most aware people thought it was too hard to play Yoshi. Roy is thought to be bad because he is bad, with terrible options and matchups, unlike Yoshi who was mostly thought just to be too hard. Parry is unlike counter regarding reaction time due to being active pretty much immediately and being used as a non-commital bait, and all the rest of Yoshi's techs were just about putting in a lot of practice. Yoshi also is not going to lose a stock even if you screw up 10 times, so Yoshi has way more room for error than Roy. And to say there weren't any really good Yoshi mains and/or players completely ignores Vectorman and Leffen, the former of the two has gotten good results with Yoshi for quite a while.
I know Roy is not good, but he's not garbage like everyone always nonchalantly says. He's still got huge disjointed hitboxes and can take on Fox and Falco. Can't say that for many other low tiers.
Roy does about as well against Fox and Falco as most other bottom/low tiers, because the problem with low tiers often stems from their matchups against floaties or Marth/Sheik as opposed to spacies, who many characters can 0-death anyways.
Roy can not compete with floaties at all, as he can never KO them if they platform camp and/or space well, which is entirely unlike basically every other character except maybe Bowser who's just inherently atrocious.
Oh Falco's and Fox's Down-smash only hit on 5 frames out of the 49 frame animation. OMGZORs the move is so useless and laggy, right? LULZ. What a dumb move!
They also come out way faster and will hit an opponent who does nothing, sometimes shield-poke someone who shields, beat someone who grabs, and still trade-with/beat a lot of moves (which was counter's only saving grace).