Yeah, that's the biggest issue. And it all originally stemed from phone carriers wanting to lock customers into their services.
We need some pro-consumer regulations on hardware which mandate open platforms. Fat chance of that happening, though, as the likes of both the EU and US want these locked down systems so they put in mandatory backdoors.
Google's own phones do not have a locked booloader. You can buy a Pixel and put GrapheneOS on it in like 10 minutes. But basically no one does this, because no matter what people say in online forums they actually value ease of use and shiny features over privacy and software freedom.
Even if locked bootloaders weren't a thing, not being able to just buy a phone with an open Android pre-installed means it would get relegated to the Linux Zone, with a whole lot of "security alert" and "device not supported". Also, low popularity leads to fewer development resources, so it would probably suffer from lack of polish.
People will keep using the OS their phone comes with and that would be Google's Android. It's worse than with Windows PCs and Windows to be honest because phones have a locked bootloader.