It's my understanding that the income of top professional players in some segments (fighting games, for example) has largely shifted to non-prize sources in the past few years. Being a top player will get you sponsorships and notoriety which can be leveraged into other projects like online shows, consulting jobs, etc. Sponsorships at minimum include travel expenses, but often will pay for full-time living expenses.

Twitch is a not-insignificant part of it, along with youtube etc. The companies that sponsor players use the hype to get eyeballs on their own sponsors' ads.

I am sorry to admit I don't have any sources for this information. It's mostly hearsay, so take it with a grain of salt.

The point is that it existed before Twitch. It might even be argued that the switch you speak of occurred because of Twitch.