I've heard that classical music played with period instruments can be pretty bad. Maybe that's why people like Stradivarius were so valued. Maybe music of that period began sounding much better.
Of course, they're still better . . . for the time being. Unfortunately, wood has a useful life, it's measured in hundreds of years but it DOES wear out. My mother in law gave me a book that is a survey of some of the great instruments, and the author noted that there is a horizon line beyond which they are going to start to fail. Some of them are preserved in nitrogen-filled cases and never played for precisely that reason. Which is a shame.
But the average quality of violins continues to improve. Even the mass-produced Chinese ones sound remarkably well for what they are.