There was some point around 15 years ago when it was nearly impossible to download and install Windows software without getting some extra adware and etc. This was true even for 'legit' vendors like Sun and Adobe. (Plus Google would offer up wrapped installers for Firefox, OpenOffice, etc.) Honestly if you thought "things were fine", you were ignoring the Linux/Mac people laughing about it.
"Nearly impossible" is quite a stretch. While it was certainly shameful that it ever became as mainstream as it did, it was a matter of unticking checkboxes in the adware installers, and there was plenty of software out there that did not engage in that behaviour to begin with. At any rate, I didn't say anything about operating systems. You can also install software of your choice on Linux or Mac. I'm not really sure what point you were driving at there.
Point being the only real difference between Windows and Mac was marketshare. (Linux doesn't have an ABI, software predominantly comes from the 'store'.)
Having an app store does literally nothing to prevent adware. Almost all apps currently on the Apple app store or Google play store are adware.