So reading the above, a few thoughts.

1. Some have pointed out a "race to the bottom" as hotels have consolidated. We've certainly seen it with airline consolidation in the US. And travel demand is strong right now, to the point where I don't have a snowball's chance in hell on getting an elite upgrade on UA as a Silver elite, unless I'm going to fly at odd times. (I was bemused to see an E175 EWR->YYZ sunday with 5 empty F seats and only two people [cleared] on the upgrade waitlist. I wasn't on it, family member was).
2. Marriott's product is ultimately to convince the franchisee to carry their label and adhere to their brand standards and pay their fees, but that's dependent on booking rooms and the value of that brand to get people to stay at a location that might otherwise pick another hotel (similar price but nicer, similar hotel but cheaper, more convenient location, more expensive hotel but elite perks help offset, etc.)
3. Marriott has been bad at enforcing brand standards. I think it might be a deliberate blind eye after COVID to please properties, but hotels play fast and loose. Nobody greets me by elite level (other than "thank you for being elite"), nobody asks "are points OK" or what my preference for a welcome gift is. I haven't had a mobile check-in processed before actually showing up to the front desk since the start of COVID (which was a great way to chance an elite upgrade, particularly if you checked in on a Sunday for a Monday stay start date). Even before COVID hit I had hotels give me a hard time over 4pm checkout before I went "I'm titanium, this is guaranteed upon request".
   (I think part of the problem is that unlike the lounge being closed or the ultimate reservation guarantee, there is no guarantee for the above two benefits not being delivered, and thus hotels just ignore them). Others cite lounges still being closed (although I'd note another part of Marriott has taken a dive here - the website for the last hotel I stated at said the lounge was closed. Much to my surprise, it was open. Front desk stated that they had contacted Marriott several times and it had been many months but Marriott can't do basic website updates for their property page on Marriott.com.)
4. To point #3, some of us may be overestimating our importance and the size of the elite pool. Maybe there are way more Plat/Titaniums than there used to be and I have DYKWIA syndrome, but the lack of acknowledgement by any front desk staff on status level or asking basic questions about my options like they used to (do you want the points?) makes me wonder if they're even recognizing it. Of course if someone is a frequent guest of a specific property they should expect better treatment, but in general expecting elite treatment to at least follow the terms is not unreasonable.
5. Regarding rates - airlines clearly publish what rates are eligible for what (upgrades, point earns, rules of the road). Marriott went this route in the SPG merger by making most OTA rates ineligible for elite benefits. Even if a rate is at a deep discount (general online rate at $600, deep discount corporate rate $199 for MoA JW Marriott), it shouldn't affect elite treatment. Or if it does, it should be called out. Marriott does not state in its terms that upgrades are subject to a United style list of status tier, then fare bucket. Just says complimentary upgrades if available. And the corporate travel rate argument is actually counterproductive. As an elite I'm more discerning about hotels property when it was remodeled etc. than most, and I don't have a choice to book a more expensive rate - my travel tool enforces that. I can choose where I drive my traffic to.

Marriott's analytics and spend algorithms and the rest may know more than I do, but for a personal calculation I'm left at a few points:

1. Marriott's award chart changes in the merger hosed me, and while earn per dollar spent is marginally better, a lot of properties became a lot more expensive. This makes staying in the points ecosystem seem much less worthwhile.
2. I used to stay at properties that were a substantial distance from where I intended to visit, to stay within Marriott, or to stay within Marriott within a nicer property. I used to drive 45 minutes each way from Ypsilanti to Jackson to stay at an FS Marriott. The Residence Inn near Jackson was 15 minutes away, and other hotels existed. But I kept my business there. And for the few weeks I was there, the staff made me feel very welcomed, starting with the first stay.
3. If Marriott eliminates key differentiators - like not re-opening lounges, or not allowing late checkout - then why shouldn't I be a free agent? If I can get a better property for less money, why should I care about elite status? Even if it's OPM for work travel, why wouldn't I try to get a nicer hotel for less money?
4. Others have correctly pointed out that some amenities have crept down (I've been to Springhill Suites that have bars) which puts further pressure on FS properties to differentiate themselves in some other way, or lest the more "upscale" properties risk losing traffic to LS properties (near my employer HQ the CY is much more convenient than the FS Marriott, without the lounge and property treatment it isn't worth the extra commute, ignoring the cost because OPM.

Marriott has to decide what it wants to do in terms of elite recognition, but they are definitely faltering right now and I'm starting to lose faith that my loyalty to Marriott is well placed, and that I'd be better off finding the nicest property I can reasonably expense in an area rather than the nicest Marriott property. Without the elite benefits making that loyalty seem well placed, it seems like I'm wasting my time and effort - even if the overwhelming majority of my stays are not on my dime.