This is already happening. For example, I've found very recently that some saver-level TATL J awards on TP metal were available from both AC and AV, but not from UA.

Every airline is, or will be, looking after its own interest sooner or later. Many airlines have restricted their saver awards to its own program, or a small number of their favored partners in the past, but UA seems to have gone the furthest among major airlines. It probably makes sense for UA since most UA miles are generated via credit card spending than flying these days. That credit card spending won't continue unless cardholders have some chance of success in redeeming those UA miles for somewhat attractive awards. Kirby is probably betting that UA is the least dependent on its alliance partners.