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Asking for a six foot friend who is trying to justify to himself the purchase of a serotta ti 26"
He has already given up on optimised performance if he's considering Ti, making it 26" too isn't going to make things much worse.
It really doesn't matter what size wheels you have if you're just MTBing for the lolz, the small differences are only relevant if you know what courses you'll be racing on and need to cover them in the shortest possible time.
The only slight problem in the long term is that it's probably going to get harder and harder to get nice versions of some kinds of tyres in 559, as that size disappears from the high performance XC/trail/enduro market. There will always be crappy 26" XC-ish tyres for the massive BSO market, and probably some very specific niche tyres for jumping and DH, plus fatties obvs., but take a look at the mainstream tyre manufacturers' new model releases - many are 584 and 622 only.
The thing that's wrong with 26" is that 29" is definitely faster on an XC bike, particularly the way race courses are prepared. The trouble with 29" is that it's just a bit too big for small people. 27.5" allows manufacturers to rationalise inventory with a one size fits all (medium height people) solution, at the expense of not giving anybody outside the 20th to 80th percentiles the wheels they actually need 🙂