>> I also pose this as a completely honest question, as I don't really know if you wiped out every desert ecosystem with solar/desal/etc if it'd actually affect anything else.

Presumably, you believe that there is something amongst all that "anything else" that has intrinsic importance. Perhaps for you it is humanity in general. Perhaps just yourself and your own family.

Whatever something you might value intrinsically is fundamentally arbitrary. Why not me and my family? Why not other great apes besides humans?

Arbitrarily, I value (desert) ecosystems as having value in their own right. Biodiversity is an intrinsic good, with no further justification required.

it's probably a good starting position to assume that since we all the share the same closed-ish system called planet earth, there's interconnections between different systems. Certainly the border areas between desert and not-desert aren't very crisply defined. Certainly (reference in other threads) nutrients can be blown by the winds from desert into non-desert areas far away. Certainly there exist some animals who go in and out of desert regions (birds, butterflies, ...). It's a really good idea to assume that things on this planet are connected to each other.

Separately from "is this desert ecosystem useful", I would assert that the biodiversity of stuff that survives in arid environments is immensely valuable, representing umptillion creature-years of evolutionary R&D.

Even if our descendants committed to turning Earth into a uniformly-lush garden world, think of its value in terraforming other planets, or even just novel biochemistry and adaptations.