I can't imagine Marriott dropping lounge access for Platinum Elite (50-night tier) and above.
It's a terrific incentive for guests to achieve at least 50 nights per year — even if those guests could have saved money by staying at hotels not affiliated with Marriott Bonvoy.
On top of that, it's a relatively inexpensive way of providing a breakfast benefit to frequent guests. Platinum Elite (and higher) guests can pop into the lounge, grab a few hot and cold items, and serve themselves a cup or two of coffee. It undoubtedly cost less to provide than the real restaurant breakfast.
In other words, lounge access is a cost-effective part of frequent guest marketing program. It's a great way sell more rooms at higher rates.