Yes I agree with your analysis: mass adoption lowers the average quality of offerings on a platform. There's even a name for this: Eternal September. Personally I've seen this happen on AirBNB, Uber, Groupon, and Facebook - yes i was on facebook when it was college emails only.

I'd like to take your thoughts one step on the "blame probably also lies with the type of customers it started to attract after becoming mainstream". I see this as a variant on Goodhart's Law which states when a measure becomes a target, it ceases to be a good measure.

For an example: high schoolers doing community service were seen more positively in the selective U.S. college application process. Makes sense if a teenager is willing to help out for free, they are probably pretty mature, ambtiious and organized, which are good things for your student body. But when the benenfits of listing your community service became known to the general population, suddenly every student was "doing" community service, but of course not of their own drive, but for the boost of their application which made it a useless selection metric.

Back to enshittification of platforms: I think the early adopters are usually selling slack that appears by happenstance. AirBNB is a great example because many people had built a guest house for visiting relatives and friends without knowing they could make extra money by renting it out online, so they built it for quality and for character, not profit maximization. But now that concept of airbnb is known, many real estate investments and remodelling plans occur with the rental opportunity in mind. In which case the incentive is to produce the minimal viable unit which can get booked at the highest price.

So in the early days you sell slack on boutique offerings: AirBNB's rented slack guest houses, Uber was renting slack for a designated driver who had no plans that night, Groupon was putting butts in seats during the off season. But then at mass adoption the platforms create a baseline of demand which exceeds the available slack and becomes fulfilled by a commodity service built for profit maximization.

To wrap up, yes this is where the profit motive drives the enshittifcation but it's not at the platform level (as you correctly identified) but at the participant level once the exploit becomes well known.