There’s a rather strong counter to this is: I, and the vast majority of computer users, don’t want to manage every nitty-gritty detail of the security and privacy of my computer. I have better things to do. In fact, doing it fully competently is more than a full time job. So farming this out to a third-party is a necessity, for me and many, many others. Of course, it’s not perfect. How much can I trust the third part…
People are lazy. They want their computer to show them pictures on Facebook, funny videos online and run some numbers for them. People don't want to care about stuff like privacy, because thinking gets in the way of fun and ease.
Malware has already bypassed Apple's signature mechanism and has been vetted to run by Apple as a result. The security and trust argument only works if Apple's security mechanisms work.
The vetting Apple is doing here is a solution looking for a problem. People have been installing third party applications forever, the ability to do so is a feature, not a bug. Instead of fixing the lacking antivirus software built into macOS, Apple is branding third party code as dangerous and untrustworthy.
I don't know anybody who cares about the problem this is solving, because it's a theoretical problem only tech literate people care about; the same tech literate people are also the people who run into this stuff the most. The reason that no one is offering a better solution is that there isn't a real problem that this solution is solving.