Arguably the phones with open bootloader have the best Android experience out there (Nexus line), so I'm not sure if Android here is the best counterargument.
They have an open boot loader because Google sells them, and they have the best Android experience for the same reason. The open boot loader doesn't enable that experience.
In fact, I think that somewhat proves my point, it's no coincidence the hardware Google has the most influence over has the best experience.
And it's no coincidence they gave up on just influencing the phones and went as far as manufacturing a phone with the Pixel, all in chase of the perfect experience