Why would the proportion of high quality games increase? The number yes, but I expect not the proportion. Lowering the entry barrier means more people who have spent less time honing their skills can release something that's lacking in polish, narrative design, fun mechanics and balance. Among new entrants, they should number more than those already able to make a fun game. Not a value judgement, just an observation.

Think of the negative reputation the Unity engine gained among gamers, even though a lot of excellent games and even performant games (DSP) have been made with it.

More competitors does also raise the bar required for novelty, so it is possible that standards are also rising in parallel.

We had shovelware games 25+ years ago (and probably 40 years ago, though I suspect the lack of microcomputers limited that). There were bargain-bin selections (literally bins full of CDs) that cost a few bucks and were utterly shite. I suspect the target audience was tech-unaware relatives who would be "little Johnny likes video games, I'll get him one of these...". Most of them were bad takes on popular games of the time.

Unity + Steam just makes this process a bit easier and more streamlined. I think the new thing is that as well as the dickwads who are trying to rip people off, there are well-intentioned newbie or indie developers releasing their unpolished attempts. These folks couldn't publish their work in the old days, because making CDs costs money, while now they can.