Firstly, we've already got a couple announcers we could bring back. Gangplank, Star Guardian Ahri, etc. It would cost Riot very little effort while potentially making a decent profit.
Second, there are plenty of big names that would be so enjoyable to listen to while we play. Gordon Ramsey would be my first choice, but don't think he would be interested. Plenty of League personalities would be happy to provide their voices, though. I know for a FACT Tyler1, Imaqtpie, Iwdominate, Yassuo, and several others would be incredibly entertaining to have as announcers.
Third, as mentioned before, it would be an easy profit for Riot as well as not taking much effort to produce. I would gladly pay to have custom announcers.
Finally, MORE PEOPLE WOULD WANT TO PLAY THE GAME! Who doesn't want to be able to listen to their favorite League streamer yelling at them while they play? *Crickets*