I agree with you, and I'm still riding my old 26" bikes. But 26" wheels have been made "obsolete" by mainstream manufacturers. You aren't likely to find any decent new bikes that use them. But there are tons of old MTBs out there that are just as good as they were 20 years ago.

If you want something new, 27.5" (584) is pretty close to 26" (559) and that small difference wouldn't stop me from getting a new bike, if that's what I wanted (I don't.) Be careful about "upgrading" with a newer bike. Depending what you have now, a new bike in a reasonable price range might be much heavier than your current bike.