Might as well get usable DNA out of them now before they disappear to the ravages of time.
That's how we do it at the State citrus park here in SoCal.
Cloning doesn't always work. You might be able to get to a callus (undifferentiated mass of cells), but moving from that point to a something that looks like a plant, with roots and leaves and that can grow in soil requires a lot of trial and error. Also money; field botany gets almost no grant money. In fact, the NSF just recently decided to end dissertation improvement grants for most areas of field biology.
Best DNA comes from before about 500 years of age. After that, degradation is pretty bad, and while we can get an idea of what it once was, we cannot currently perfectly reproduce it.
Yeah, that's going to work fine. This plant will never procreate without explicit human effort. We can do this for a few species of course, maybe even a few 1000. But it doesn't really change the fact that the species is extinct. It can't survive in nature and humans "saving" it for centuries won't change that, it will make it worse.
We cannot seriously hope to recreate the entire environment these species depend on. We can only save the DNA. Cloning by itself is useless, in that it can only create a few new individuals that then proceed to die in a very unfriendly environment.
You'd say that but then there's a SINGLE species of lizard in Australia which depending upon where on the mountain it grew up it either creates eggs or bears live young.
You think you know genetics but things like that prove very much that we have ZERO clue.