The whole size thing is just another example of marketing push. The big brands get together and decide on what new specifications and standards they can come up with to make your current bike obsolete faster so you buy newer sooner.
It's no more than that.
Did you watch and understand the video? Santa Cruz say it is customer led.
they hit a ridiculous bar with 29-ers. Not many people fall for it,
Did you watch and understand the video? Santa Cruz say 60% of their sales are 29ers.
So if you like having to accelerate greater mass more often during every ride, go for it, knock yourself out buy some 29's.
That would be true if everyone was running the lightest wheels and tyres and tubes / tubeless systems all the time. But they aren't. So sure, for the exact equivalent wheel / tyre / system, the bigger one will be heavier. But if you upgrade your wheel as well as moving to a larger size, the difference cancels out. Likewise by choosing a lighter tyre, lighter tubes, or whatever.
Also, you'd think the people that would really notice if there was an efficiency problem with 29ers, like for example the cyclists who do the Tour Divide, they wouldn't choose them. But they do, all of them for the most part.
protect your investment
I dunno what is to protect. You have you bike, go enjoy it.
There is a lot of worrying about stuff that hasn't happened yet. Sure, frame and fork choice for 26ers will get less, but you'll still be able to get them if you really want them. And if 650B does really take over and kill 26ers totally, well, your next bike will be a 650B. Sure, it is possibly added expense because you can't use your old tyres, but that's about it.