Not necessarily islands. Plenty of contiguous large ranch areas still. Mostly South of I-10, Anahuac to the Sabine River. But plenty of others also.
There were plenty of droughts and cold spells prior to 1980.

But prior to the 80s they were connected to other populations. I would be very surprised if there is hundreds of thousands of acres there still entirely in native grasses etc. and there probably are still some birds there on those ranches but they're just not on the small tracts etc

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Like every other thing, it is undoubtedly a combination of multiple factors and I bet every single one mentioned on this thread plays a pretty noticieable and measurable role.

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The thing that concerns me the most according to this podcast-- on pristine habitat where the parasite is present quail are vanishing. Not because of development, habitat fragmentation, hogs, coons, hawks etc. It is because of the parasite as evidenced by the experiment run with controls.
Pristine habitat=native grasslands on ranches with tens of thousands of acres.

If you can't keep quail numbers up on the best habitat on earth for them because of the parasite, the threat should not be minimized.

The eye worms are not a big problem in South Texas quail, according to a study done by the East Foundation. Biggest problem, according to them, is rainfall and habitat change.

https://eastfoundation.net/media/management-bulletins/2023/management-bulletin-no-9-south-texas-bobwhites-and-eyeworms-regional-history-prevalence-and-implications-for-management/