I honestly think that 29″ will be pretty much standard for medium and up frame sizes. 650b for smaller frames.
I don’t think 650b will totally go the way of 26″ though.
I think you’re probably right.
I don’t really keep up with the latest bikes so much, but the two friends who most recently bought bikes bought 29ers. I just had a quick look for 2020 bikes, seems a decent mix of 275 and 29 and yes a couple who offer 275 on the smaller frames and 29 on larger which makes a lot of sense I think.
TBH I’m way past worrying now, 26 “died” in 2013 (2014 year mainstream bikes were all 275 or 29 bar Spesh) but you can still buy tyres, tubes etc, even forks if you try hard enough, that’s nearly 7 years ago now, and enough riders still seem to feel the need to tell everyone their 26ers still work.
‘The Industry’ seems more than willing and able to support ‘old’ standard long after they stopping selling new ones so if 275 “dies” I think we’ll all be just fine, but I don’t think it will. Seems we can have 2 wheels sizes at the same time as the Earth still turning.