Teambuilder had a lot of issues on its own and Riot does not want to go back to it. Instead they need to look at moving forward how to resolve the problems. Like what did Teambuilder do that we liked?
Well for me Teambuilder was great because for the most part unless I wanted to try something way out of the box and no one wanted to play with me. I could in theory play any champion I wanted at any role. So I could guarantee that I could practice like Vayne ADC. There would be no risk that someone else would pick the champion, ban the champion, or take the ADC role.
The problem with teambuilder overall was the fact that it still required people to lock you in, but even more importantly no one wanted to play support and the queue times could be insane.
Fastforward to today Riot tried to create a compromise, but it didn't work. Now you can not guarentee you get your role or champion and you still have the queue times before of the small percentage of people who play support. (for good reason people treat them like poop. Real supports are a smaller number every day)
Riot needs to try again.