Environmentalism is often just a stated preference. Like humans shouldn't disturb some thing.

So it isn't necessarily the case that you need to be more knowledgeable to challenge them. Availability of inexpensive energy without burning coal is another preference.

I certainly have reached the point where I think popular environmentalism is waaaaay too focused on static conservation.

That's a generic view which I might actually be inclined to agree with you on. I'm personally skeptical of the current climate change politics, for example.

But OP made a more specific claim about solar arrays in the desert. Seems like we should be able to debate that on its merits.

That alternative to putting them in the desert is putting them in some non-desert area or putting them nowhere. If these people are objecting to putting them in a desert, they'll object to putting them in a non-desert area too.

Making better use og roof-tops generally is a win as far as environmentalists are concerned; NIMBYs/BANANAs may complain however.

Not that I see any intrinsic beauty in concrete titles or roofing felt. Even clay tiles and slates I suspect are just something we have gotten used to.

In all cases they are a financial and embodied energy and financial cost as-is, and PV can turn them into something positive.

Ignorant people look at a desert and see lifeless desolation. When they educate themselves they come to see the desert as an very active environment with limited resources, and cut-throat competition for same, that makes human impact more pronounced. Better to put solar panels over scrubland or even farmland than into deserts.

Generally, it isn't really environmentalists exactly fighting these projects. Instead it's folks who don't want them near where they live or other powerful interests in the vicinity, who utilize existing environmental regulations to put these projects into analysis limbo for years on end. So there is a relationship in that the environmental laws are able to be abused to stop NIMBY projects. The same laws are now being used to stop High-Speed Rail.

I'm guessing theyre sad because they don't value desert habitats highly nor appreciate them and are probably raised to think deserts are "empty" when thats far from the truth.

I think some reference is useful, here.

Agriculture is and will be much more land-intensive than solar.

I once calculated that the US could switch entirely to solar just on a fraction of the land that is currently used for ethanol production alone.

And what is a better place to put solar panels? In the desert or fertile arable land? Deserts contain life, of course, but the density of life and biodiversity per unit area is much smaller. Additionally, solar panels provide shade and, unlike agriculture, don't require intensive tilling, harvesting, irrigation, spreading of pesticides (natural or otherwise) or fertilizer and the runoff that causes. They mostly just sit there, maybe getting a cleaning every once in a while (but unlike concentrating/thermal solar, don't need to be cleaned super often).

Once the panels are placed, your desert tortoise can nestle in the convenient shade they provide.

We'll destroy habitats anyway. But stopping/slowing down climate change by reducing the habitat of some species will still be beneficial for global diversity of species. I find it hard to believe that warming oceans would kill less species than solar panels in the desert. And even for 100% renewable energies, only small parts would have to be used for production.