This is a good point; we shouldn't act as though users are necessarily making an informed choice or meaningfully consenting to all the software that's on their computers. Lots of people are forced to install software at economic gunpoint (and probably can ill afford a separate computer to isolate it on).
You can't depend on users and the marketplace to select against insecure software. The market is too distorted to function that way; the people forcing others to use shitty software are often isolated from the consequences themselves, so there's no effective feedback loop to stop it. Having the OS vendor step in is really the only good solution in the short term.