It's not a regret. Android would never have been popular in the first place if it had not been open source. If phone makers had been able to anticipate how much the demons at Google would be able to lock down the Android then they would never had used the OS back in ~2009.
That’s possible (never underestimate the bad decision-making of phone makers), but what would they have done instead? Windows Mobile 6 was the only licensable alternative, but it was a known quantity and obviously a generation behind Android and iOS.
No, everyone knew what they were getting into with Android. It wasn't taken lightly by the power users of the time either. I still have a phone somewhere with Ubuntu Touch on it. I really was hoping that would be my phone OS by now. Not that Canonical isn't capable of similar, but that it would bring about acceptance of Linux phones.
I think we're all more surprised by how long it took for Google to make these bad moves. For a period of time in the 2010s we actually started thinking maybe Google was alright.
Phone manufacturers had Symbian which became open source
I don't think this is actually true. Phone makers seem happy to have the platform locked down even further. They're putting even more roadblocks, as evidenced by most of them making bootloaders unlockable.