Slightly dramatic thread title. I think your post assumes a lot; I personally am very much looking forward to fan run legacy tournaments and am of the opinion that extra formats are a draw, as in mtg.
I think there are issues with bringing players into the game, but those are helped as much as hindered by rotation (more imho, smaller card pool). There are also plenty of other things that could be done to help with these issues, e.g. cycle packs.
As for a reboot, I’ve long been of the opinion that some form is inevitable. The core game is too unique to just drop (and not particularly dependent on any IPs remaining popular), but as the core mechanics don’t appear to be broken my money is on a ‘soft reboot’ similar to what games like Magic have had. Basically Core 2.0 and maybe fast rotation for a couple of cycles. This is in no way necessarily a bad thing.
In the short to medium term, I’m optimistic about rotation and am hoping for a best of both scenario with the appeal of a strong legacy community (and the op combo gameplay that brings) coupled with the benefits of a smaller cardpool for standard play.
I guess we will see, but as long as the design on the cards stays solid, I think we’ll do ok. Practically speaking, the best thing we can do is to not to be insular as a community and to value newer players. See the excellent article that I’ve forgotten the location of but would love to link.