It seems like quality lowered over time for several reason. 1) free products became popular, reducing revenue, 2) increased automatic updates and better Windows safety models reduced need and 3) Windows defender became decent around Windows 10 so other offerings became redundant.
it's not just about windows defender becoming decent (although that's certainly a prerequisite). the motivations are more aligned in favor of users when OS providers are also the AV providers. an OS provider wants viruses to be eliminated, because they harm the OS' reputation. a third-party AV provider wants viruses to continue to be a problem, because consumers' fear of viruses drives their revenue. all other factors being equal, if i'm using a microsoft OS, i want a microsoft AV.
