Alright.
Agreed. I'd imagine most players would be better off picking Lucina. Even Mr. E, a top level Marth player, seems to prefer Lucina in many MUs (then again, Lucina is somewhat better than Marth in some MUs).
Conjecture. Mr E's success has seemingly only gone downwards as he's upped his usage of Lucina instead of maintaining his Marth.
He may find her more comfortable whilst also having a more concise victory condition, but the latter goes both ways, especially against a fundamentals spacing character like Marth and Lucina.
P.S. One day I really hope people stop acting like tippers are some sort of random mechanic.
They've somehow been able to not be a random mechanic in two previous iterations of Smash.
Corrin's up-air, bair, neutral-B, up-throw, and side-B are all better than Marth's. In general, I'd say Corrin has a somewhat easier time killing than Marth, though Marth does have some pretty good options (like jab to f-tilt, or side-B near the ledge, or backair, or tipper f-smash).
Corrin's bair? Better at killing? Maybe in some raw-numbers only world? Better in general??? The bias here is unreal.
Marth's bair is a huge move that tippers close to his body on start up. In this match up in particularly this move punishes and beats out essentially all of Corrin's zoning tools (including pin).
Also hilarious that you forgot almost every move Marth has kills.
Being literally better in some sort of vacuum universe is a shallow way of arguing.
Corrin's up throw is better at KOing, but is it easier for Corrin to get a grab?
Pin is an amazing and centralising tool, but holy moly, Dancing Blade is one of the best Smash moves of ALL TIME. They exist in slightly different paradigms where the main difference you seem to focus on is one is overtuned/overloaded while the other is not. Both are bread and butter to them. However, Marth would still function to some extent without Dancing Blade (although prior to them fixing it up his status as a low tier was an easy observation) while Corrin would barely function as a character anymore.
He goes even with Corrin and goes even or slightly loses against Cloud. It's not bad, but it's n.ot exactly "good counter-pick".
I'm not sure Corrin is even; Marth's sword is effectively bigger and his bair and nair push through the zones Corrin's entire game focuses around.
But to be fair I rarely see this match up played at higher+ levels. However, Marth by design naturally excels and overwhelms the FE cast one way or another; Corrin's "brokenness" can surely even that up a bit, but fancy losing footsies, losing in disjoint and not being able to recover? That's what Marth does to basically all the Emblemers.
Marth "kills much earlier"? I'm not sure if I really agree there. Anyway, it's true that Marth is faster, but Corrin is heavier. "no horizontal recovery", are you forgetting that Corrin can use her bair to assist her recovery, if needed? And her up-B does travel quite a bit if you angle it. Corrin's can also hit through the stage for some stages (like Battlefield) if you angle it correctly.
You mean, using bair, in the opposite direction of where it's hitbox is that's hugely telegraphed?
People aren't really edge guarding Corrin yet either - the character's up-b is slow start up and they're completely exposed to the side/below them.
If you cover for/take away their option to mix up recovering high, traditional smash edge guarding (i.e. timing a dropping from the ledge -> aerial or jump aerial) is very effective against Corrin - not vs Ganondorf effective but people are horrendous at trying to end stocks off stage in Smash4, Corrin is a very susceptible case.
On the opposite end, edge guarding Marth is underutilized too, but not too many characters have the right type of aerials for it.
Ledge drop bair from Mario will trade almost every time but not always favourably (Sheik has it even worse; but there are some weird sweetspot percent where sour dolphin slash trading with bair gives her a very strong BF follow up likelihood), yet Doctor Mario does it better because his bair actually strong enough to stop Marth from being able to come back after one success. Ironically, again, low tier power, but Falco's down air being a large hitbox with long duration AND low end lag (after the hitbox ends) means that he can either spike Marth or get a sour hit into back air (or fair).
Traditional edge guarding is harder on Marth depending on the stage (you want space in front of Marth), but while Falco or Zero Suit can't just ledge drop bair (usually; for Falco the risk of a trade is still worth it), them timing a ledge drop and rising up with their bairs against Marth forced to up-b is a dead Marth.
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Now for the rest of your post, those 3 match ups are likely
easier for Corrin. Primarily because of the power of certain tools.
Marth doesn't have any exuberant tools like that for sure. He's a lot more smash-skill based than abusing tools based.
But he definitely has more diverse tools.
So who is actually worth more in the meta?
Well if you only focus on match ups in such a way that revolves around the super-strong tools and neglect most of everything else, Marth's going to look weaker.
Top 10 area is a lot more evenly matched, barring Bayonetta, than people tend to give credit for IMO.
Corrin could come out on top in the long run, but Marth will always have more options at his disposal.