Just because there are known vulnerabilities don't mean we should drop other security features, this is the opposite in fact. Defense in depth, an OS-level vulnerability cannot be exploited if the attacker cannot access that part of the OS.

And like it or not, the Play Store approval process is a security feature. It limits the ability of bad actors to run code on your phone and access data or exploit vulnerabilities they wouldn't be able to otherwise. Some get through, but it makes their life harder, again, defense in depth. Something can be both an anticompetitive practice and a security feature.

As for banking in the browser, you can, but your bank probably doesn't like it. That's why they are pushing for browser attestation, or to force you to use the app. The banks would rather take that freedom away from everyone rather than giving it to everyone. And I suspect they do it for good (as in profitable) reasons, fraud costs them, it costs them more than what they would gain by being more open.

If we want security features and freedom (which is the harder option), we need competition. If Google and Apple are the only players besides an insignificant minority, it is easy to lock software to these platforms, screw that weird guy with his Linux distro. Legislation is another option if the first one fails.