1) The Democrats simply cannot be trusted to keep any bargain. There will always be "exceptional circumstances" when they're in danger of losing.
2) Therefore, the Republicans are just going to have to do it all over again as soon as there's a Supreme Court vacancy.
Of course, if the Democrats obstruct a Supreme Court appointment coming so soon after this compromise, they'll reveal their naked obstructionism and the Republican caucus (so soon after having swallowed this compromise) should rally and vote them into oblivion. The sooner the better, I say.