Then they would have basically no way to compete with one another other than marketing and specs. Touchwiz and all those other gimmicks really do sell phones. That's just speaking from anecdotal evidence, but I assume big corps have done their homework on this one. Remember that the types of people reading this comment are not your average consumer. We might hate it, we might know it's not as good as stock android... but we're a very small segment of the market (especially for Android). You probably don't see a ton of nutritionists/personal trainers eating at McDonalds, either.
The downside of having an open platform is that OEMs will add crap to differentiate themselves - just look at the weird media bs that comes pre-installed on tons of windows laptops.
I suspect this is temporary. A lot of the big OEMs from just a couple years ago are massively losing money on Android. HTC, Sony, LG, etc all reported signficant losses. I imagine the market will tigheten up with one or two premium OEMs (Moto, Samsung) and some bottom feeders selling super budget phones. When things finally tighten up there will less incentive to make these difficult to maintain Android custom distros and more incentive to get closer to the stock as a way to make updating easier. Google may also put in a full theming engine to help this along. Marshmallow has a basic theme engine form what I've read, so that's a start.
I think Google knows that the status quo with Android is unsustainable and will need to migrate to a more tightly controlled business model. Samsung's flirting with Tizen hasn't paid off and they're more or less stuck with Google dictating policy now. The monthly security updates thing sounds promising as well. Its time Android got serious about security and updates.
The downside of having an open platform is that OEMs will add crap to differentiate themselves - just look at the weird media bs that comes pre-installed on tons of windows laptops.