Define significant. Session musicians were definitely a thing that earned a middle-class lifestyle during the last century, at least in larger markets. Of course, samplers and sample libraries likely had a larger impact on reducing the viability of that lifestyle (no need to hire everyone necessary for the string section on these three tracks when Kontakt and the one intern who knows his way around automating expressions will do), but shrinking studio budgets due to the impact of piracy helped put the nail in the coffin.