One of the biggest complaints, all generation long, has been the wide range of offensive threats that makes defensive team building a challenge - Ogerpon-Wellspring, Kyurem, Gholdengo, and Zamazenta all have very different counterplay, with nothing reliably covering three of the four, never mind covering all of them. Stall can make it work, but it takes all six mons to do it reliably, so even fat balance can struggle to have a defensive answer to everything. At the other extreme, HO also makes it work by answering everything offensively instead, happily trading one mon to weaken a challenging opponent into range for another to beat.
It's everything in between, ranging from regular offense to the fattest of balance, that can't cover it all with a three or four mon backbone. There's just too much variety in attackers.
A centralizing mon or two can actually help with this problem. Dropping Zamazenta offered a blanket check to physical attackers, being able to 1v1 everything except a handful of specific Tera Ghost mons; Volcarona, while it was in the tier, offered a blanket check to many special attackers, though Volcarona itself was enough of a problem to answer that it ended up getting banned.
Palafin has the potential to blanket check fast, frail mons by using its powerful priority and good bulk. Iron Valiant challenges? Chip it down and Jet Punch can do the last 40% even without offensive investment. Kyurem running over your team? Bulky Drain Punch does about 50%, and the set can take a Choice Specs Freeze Dry if healthy. Kingambit being scary? Encore sets can lock into Sucker Punch and then click Jet Punch, or for banded sets, Tera Fighting survives a +2 5 Fallen Sucker and is favorable to kill with Wave Crash. Ogerpon a problem? Run Close Combat over Drain Punch and a neutral Tera (again, probably Fighting for overlap with other answers) and you can take a +0 Power Whip or Play Rough.
All of these techs are viable, all of them have an opportunity cost that weakens Palafin offensively so it isn't overbearing, and all of them can check multiple threats. Palafin returning would benefit Sinistcha as well, which also covers a good range of offensive mons without being a problem itself, which might further help in adding a little centralization to the tier.
Lastly, if Palafin's reintroduction pushes a few mons out of OU by reducing their viability, that's probably a good thing - reducing the number of offensive threats means it's easier to build defensive cores, and Palafin has enough answers of its own, that are good mons anyway, that it doesn't apply too much building pressure itself.
