If Google weren't paying manufacturers to not install other app stores, other app stores would be paying manufacturers to install them, which could result in a user experience that Google doesn't control but which users would blame Google for ("Android is slow/buggy!" when in fact it's third party software that is causing the performance issues). Their deals are arguably defensible since they don't block third party app stores entirely, just pre-installed ones.
I certainly see the issues with Google parlaying their dominance in the ad market to also dominate the mobile market, but I have a feeling this will be settled with some consent decrees to put up some guardrails in the ad market. The actual harm to consumers from Google's business model is pretty vague and is passed on through marketing expenses that trickle down into product/service prices. If Google disappeared, ads would still cost money and advertising expenses likely wouldn't be drastically lower.
There are a bunch of Android forks in the wild[1].
[1] https://en.wikipedia.org/wiki/List_of_custom_Android_distrib...
That's absolutely correct. And some of those forks are significantly better than the Android OEM offerings by the phone manufacturers.
However, that's orthogonal to GP's point. Only a very small percentage[0] of Android users would even consider flashing custom roms, and even fewer would have the knowledge or confidence to do so.
As such, the impact of custom roms/no chrome is most likely rounding error for Google.
I'd point out that I've used custom roms quite a bit, initially because (as usual) my phone vendor stopped providing updates after a year and after that because custom roms are often better than stock roms.
But I'm not your typical user, as I'm technical and it really pisses me off that vendors use the lack of support to pressure folks to buy new phones.
[0] https://stackoverflow.com/questions/19943324/how-many-users-... (an old link, but it's not likely that much has changed to increase the number of folks using custom roms -- quite likely the opposite)