I just switched from a 7 with TouchID to a 13 pro with FaceID, and one of the reasons I held off of upgrading was specifically TouchID, so I have an opinion on this.
For interactive use, i.e. holding your phone in your hand, looking at it to actually do something on it, FaceID is vastly superior to TouchID. Most of the time it's done its magic between you picking it up and swiping up to unlock, meaning it's seamless. It feels as if your phone was never locked to start with, but it was. Sometimes it takes a half second after swiping to do the unlock, depending on how you were holding it. I'm honestly surprised at how good it really is, and I never imagined it would be. FaceID also works perfectly while you're wearing gloves which is important for those of us in climates that aren't warm year-round.
But: for using Apple Pay, TouchID was the superior option. There is a big difference in the smoothness of a transaction at the register if you can grab your phone out of your pocket/purse/whatever with your thumb on the home button, hold it in front of the reader, and put it back versus having to hold it up so it can see your face, press the power button twice and only *then* be able to hold it up to the reader (and if you mess up the order of operations, it's worse.)
TouchID also enabled some other use-cases like checking your phone's notifications (or some such thing) with it on the table in front of you, without having to pick it up but frankly I feel like those are mostly just habits developed because it enabled us to do that specifically because it was in the home button, and TouchID in the power button won't really change that. TouchID on the power button would still work great for Apple Pay though. And of course, TouchID works better in the presence of face coverings like masks.
So yeah my opinion is that they should add power button TouchID to iPhones, specifically for Apple Pay and situations where FaceID can't see your face because it's obscured.