Yeah I’d say the straight steerer is going to be the biggest problem rather than 26″ forks, RS are still knocking out 26″ forks. One consideration is spares for your fork, I think I heard that Mojo can’t service some of the oldest 36’s because they don’t make all the bits for them anymore – admittedly they’re 10 year old forks now. You might consider buying a small pile of seals for your forks and keeping up with the maintenance, but if they’re new, you should expect to see at least 5 years from them, disaster aside, and that’s past “foreseeable future” in my book.
As for the rest, I wouldn’t be too concerned, I’d bet the majority of bikes being ridden are still 26″ – the first major switch to 650b I think was 2014 model year bikes, not massively long ago and there were still new 26″ bikes coming out in 2015 (and still are now) we may think ‘the industry’ released 650b as a evil plot to create the idea of our bikes as being old and outdated, but they’re not evil enough (or perhaps they just don’t have that much of a cartel attitude) to actively destroy the existing ones by making them obsolete – if there’s money to be made selling bits for ‘old’ standards, then someone will serve that market.