Supporting GS flyers is expensive. Upgrading award tickets, extra upgrades, extra saver awards, dedicated call line to deal with mostly non-revenue generating requests.
The question I suspect UA has changed its view on is: how much incremental revenue do we get from that? How many of our GS' are hub captives? Unwilling to change to another program anyway? And of the ones we lose by yeeting benefits that costly? Sure, they book full fare and that's lucrative -- but what if I can sell that seat as an upgrade or a non-refundable fare where I don't have to provide squat in terms of benefits to ma and pa? How often am I prevented from doing so because the GS cancelled or changed flights close-in?
I suspect with rising labor costs and fares, the math on all of the above has changed. UA is cutting out the fat from the GS program, and thinks (or is at least testing the proposition), that the GS' who actually change carriers due to this aren't sufficiently incrementally profitable to outweigh the costs.
It's annoying as a GS, but it's just business.