Now that sounds like foresight! I think rims are going to be a real pain...
I bought an excess of rubber way back, and that’s been useful. They are fickle though, with some compounds killing themselves slowly in storage, while others sit there asleep and nice waiting to wake again. IRC seem to be excellent, and come out supple and new, even after 20 years in the dark. WTB varies, and develops deep cracks. Ritchey all over the place, with some falling to bits like a zombie in daylight, while others develop tiny tiny cracks, and others are fine. Good old Schwalbe still know that people need a variety of 26 rubber...
And regarding 29 650b/27.5 26....we run all three. When I am footling around doing distance or swooping on simple loamy tracks, 26 is fine, if not better. Jumps and pump track - 26.
A 2.5 up front and 2.3 in the back gives me a very stable and easy platform. It’s notable though, that on washboard tracks, I am slower than others on 29. The science explains why. But doing Alpine DH, I want 27.5 or 29, and modern geometry.
When about to run a section with a large drop-off, rutted run out, then a ditch and rooty segment - then 26 is SCARY!!! So that’s when a 29 with 170 on the front and 160 at the back comes in right handy....
Mountain biking has changed since the 1990s - hugely - and I am going to write about that in the mags soon. The whole approach and attitude has shifted. We used to be 26 and leave no trace...now it’s 29 down dedicated trails dug out of the hillside. If you don’t get air and a whip in then you are NOBODY. Hence the woods being full of diggers now. But Friday Fails on Pinkbike shows how weird some of it is - people doing things which they do not have the skills to do. Collarbone, collarbone, wrist, collarbone, shoulder separation, concussion, collar bone. It’s all a bit silly.