Shouldn't reefs just migrate north in the northern hemisphere, and southwards in the southern hemisphere? I know this process can be quite slow, but on a long enough time frame it's conceivable there may be no net loss and possibly even a gain global reef acreage?
A temperature delta over a millennium is very different than that same delta over a century. Coral reef ecosystems take centuries to establish themselves and mature. They can't simply grow legs and walk a couple miles north every year.
To answer your question: sure, 2000 years from now it's hard to predict total acreage, and it may well be higher (or lower). But there's little uncertainty that on the timescale of centuries, corals will be devastated, and that means really bad things for us and our children, to say nothing of the health of marine ecosystems.