When you finally build that 650b rig you’ll want the latest and greatest forks anyway. Plus you might have cut the steerer to a point that isn’t right for the future frame.
I went through these thoughts and because I needed new wheels for the new forks so I could run 15mm through axle, it was getting to the point of buying a 650b frame anyway. Then the PSA on cheap Pikes popped up and decision was made, 26″ Pikes and make the most out of my existing c456 frame that has only had cheap forks so far.
I’ll run with that for a year or two and see what happens then. My full sus is also 26″ and much life left in that yet and want to keep compatibility rather than have a mix of 26 and 650b tyres and wheels around the house.
To be honest the Pikes on the c456 have been impressive and with the new wheels it’s a whole new bike.