That's not necessarily true. There are other disadvantages to enabling voice chat. I personally would opt out every time because it is rare that I play without a premade group of 3 or more. The chances of getting a teammate that's actually worth voice chatting with seems far too small to me, and I don't trust people to not gum up our comms. It is a valid concern because it creates a negative environment from the start.
Also, I think you underestimate the amount of bandwith and ram a live chat system eats up. Some people don't have the capability to handle that kind of stress on their system or ISP, and those players can't be punished for not having access to those kinds of resources. There's also the issue of people not owning mics at all. It would be a poor business decision to make a third party peripheral so central to the success of team play. I can already imagine players flaming the support because he or she doesn't own a mic, and that automatically puts that person on tilt.
Lastly, it would be far harder for Riot to monitor unsportsmanlike or toxic behavior in a voice client because it would be difficult to both log that information and review that information. A chat system can be scraped by a bot to present unacceptable behavior, whereas voice logs would have to be reviewed manually to find any record of inappropriate behavior. For a game that already has a reputation for having a negative player base, providing open comms would only make that worse. It also provides an unmonitored avenue for scammers to solicit personal account information.
Overall, hard pass on public voice chat for me. I will stick to curse and send chat links to anybody that seems worth chatting with or those that sticks around for more than a single match.