Well, I can answer this from my perspective.
There are a lot more 5*s out there than 6*s, and if you're like me and you're picky as to the waifus you want on your team, there's only about a dozen 6*s available now that are ideal matches -- the rest of the characters I want on my ideal team are 5*s with designs I really like. Even if they don't make my top 20, they can join whale squads and kingdom-specific secondary squads and whatnot.
The vast majority of recent one-use gacha offerings, like the Christmas one or this most recent step-up gacha, have offered 6* tickets but not 5* tickets.
Right now I basically have 10 of the 12 6*s I want, but considerably less than half of the 5*s I want.
So sure, more 6*s are never a bad thing. But we'll soon reach a point where I'll run out of 6*s I even want with future step-up gachas and whatnot. More than half the available 6*s (and more than half the available 5*s, too -- there's just a lot more 5*s to choose from) have designs and personalities I'd never put in my squad no matter how advantageous it'd be to do so from a stats / numbers perspective. Only characters I like join my squad. Characters I subjectively dislike have no place in my ranks.
No doubt the 6*s are a great boon to the vast majority of folks out there who aren't spending real money for the 6* tickets in other contexts, but if you're like me, you'd rather have a 5* ticket at this juncture -- which is why I'm surprised FKG hasn't implemented a feature where 6* tickets can redeem 5* or 6* characters. "Lesser or equal value" exchanges are common all the time in other marketplaces, after all. And the characters I want most right now, from a sheer personality perspective, are all 5*s.