The "death of 26" is a straw man argument.
​​​​​​Sure, the industry and the sport of MTB left 26ers and 3x behind over a decade ago, but there's still lots of things being produced for them; you just can't walk into any random LBS and expect to find it all on the rack.
Especially, if like the OP, you're trying to find a very specific combination; as mentioned, there was very little overlap between rim brakes, tubeless tires and 26ers, so you're really limiting your options


WRT tires, Conti, Schwalbe and MAXXIS still produce a thorough selection of "proper" MTB tires; Panaracer as well, still puts out a lot of retro/inspired stuff, including the classic Smoke/Dart. Likewise, there's a lot of 26" BMX tires that do really well in the commuter/, utility bike role a lot of old MTBs find themselves in.
Also, since there were so many 26" bikes built over the last 30 years, if you happen to be stuck in some out -of-the-way place, desperate for any replacement tire; odds are, what you'll find is a 559 "26er"

As far as the "value" of upgrading any old 26" bike to 1x and tubeless, to increase the resale value; It's probably not worth it unless you've got a desirable, high-end model to begin with.
(Even then, I don't think there's going to be the "irrational nostalgia value" for early 90s Trek 820s and Nishiki Pueblos like the Boomer's have for 60's tanks like the Schwinn Varsity