Why voice chat is a bad idea
There is no reason it would be a good idea. LoL is already known as one of the most toxic games ever. Voice chat would just have people yelling at others in different languages.
Look at what CS:GO is like with voice chat... But CS:GO even has a good reason for a voice chat because it's more fast-paced and you need to give call-outs quick when there's no time to write in chat. LoL is not, and usually you have time to write in chat unless you rage and just want to flame your teammate with a super long message. It would only do more harm than good being used as a verbal harassing tool rather than a tool for call outs.
But let's have our fingers crossed and hope that it's not going to ruin the game like CS:GO did.