I suspect it's more related to the Amex Centurion Studios takeover. ONT has been pretty quiet through the pandemic - I've booked a lot of flights out of ONT that I either had to switch to LAX or change flights and add a connection to when they were cancelled, but seems like it's become pretty stable finally. I think I have 4 or 5 trips booked out of ONT that have been stable through multiple AS schedule updates now. But with Escape Lounge changing to Centurion Studios, Amex or the lounge operator might have decided that the ONT demographic doesn't include enough potential Amex Plat customers to justify converting those lounges. From checking out the Centurion Studios threads, it sounds like they're not going to open those lounges to PP, and I don't know if they have a one-off rate or special rates for gold and green card holders as teasers.

(ETA: they do have one-off rates, but I can imagine the amount of business they see for the next couple years at ONT isn't enough to justify keeping them open. Once you're airside, ONT doesn't seem to be terribly crowded or hectic, even if the flights are all full and the TSA checkpoints are understaffed.)

Travel is so upside down right now that parking and ground transport are as big a driver of the airport I choose right now as anything else. With no economy lots right now, and private lots booking up early, parking has gotten really expensive at LAX, and uber/lyft to the airport have uncertain waits and the lowest price you'll see is still almost 2x pre-pandemic, and can be much more. Economy parking at BUR was closed until about 3 weeks ago. ONT has walkable parking (even from the farthest cheap lot), but you have to face the 210 East for evening flights.