> Is it bad for Chrome to have vulnerabilities?

Yes, obviously it is. Is it bad for others/the public? Probably, but not as bad as it is for Chrome.

> because it really is only Chrome here holding the power

I'm not sure this is true. Apple pretty much forces usage of their browser engine on iOS, and heavily try to get people to use Safari on macOS. Windows push Edge pretty hard on their OS, and their browser engine is pretty much intertwined to the OS so you can't not use it. Both of them say they let you change the default, but various links in the OS would still open Edge/Safari even if you have the default browser changed. Not sure if that's on purpose or not.