OP also appears to have compared different classes of properties (e.g. full service vs. limited service) across hotel chains. Comparing IHG Holiday Inn Express (HIX) to a full service Marriott property is apples to oranges. HIX to Fairfield (or maybe SpringHill) would be a valid comparison.

I have limited data points from my direct experience so not statistically valid. With that disclaimer, the recent trend I am seeing is back towards normal breakfast offerings with both Marriott and IHG (the chains I frequent). In contrast, last Fall and last Summer offerings were limited grab n' go. With the spike in cases from the Delta variant currently in play and possible upcoming reinstatement of travel restrictions, hard to say if the trend back to normal breakfast offerings will continue. I hope it does. It is shortsighted of hotels to skimp on breakfast offerings even with low occupancy. If they legitimately need to cut breakfast services due to high cost of providing it during low occupancy, then a meaningful reduction in the nightly rate should be offered instead to reflect the reduced service level.

Have another stay coming up next week and per the MR web site the lounge is open for breakfast. Looking forward to my first post-Covid lounge experience. Will report back after my stay.

--Jon