I've owned three altimeter watches, and at this point I feel a bit of fool to have done so. The first required that I mail it back to have a new battery put in every 18 months or so, an expensive option --- so watch out for that, get one with a user-replaceable battery.
The other two eventually both got wonky on me, i.e., at some point the elevation readings just got unreliable, regardless of how carefully I recalibrated at known points.
On a thru-hike, at some point your legs get so strong that you just don't care as much (you still care, just not as much) about elevation deltas --- you'll obsess on this less. I found that I just didn't tend to look, whereas for trips when I wasn't in good shape I would sometimes find myself looking at the elevation all the time on a long climb.
If you have a smartphone with a built-in GPS (that doesn't require cell towers to work), on an infrequent basis if you really want altitude (or location) you can get it that way. And if you need it frequently then, IMO, in most cases that means that either "you're doing it wrong" and/or you would find it worth carrying a standalone GPS unit.
The other thing I like out of a watch (besides current date/time) is an alarm clock (quite infrequently). My smartphone does a much better job of that too, watch alarms often don't wake me up. In fact, one could perhaps just omit a watch in favor of a cell phone. I do like having a very light, cheap Casio on my wrist when hiking but it's not an essential.