Let me put it to you this way.
Instead of the (2+1+1+1) (2+2+1) (2+3) (3+1+1) (5) and the (should be removed) (4+1) team comps we are enjoying now, we would be limited to (2+3) and (5) premade team comps, which have to have a support in them. Which will skyrocket queue times for dynamic queue.
And since, from what I have heard(and experienced), a lot of supports hate to queue alone because people cannot be nice to the person who, in most cases, is keeping them alive, they play in 2+ premades for either support against the expectations of a idiotic adc, or to for sure have an adc with realistic expectations. This, of course, would lead to a deficit of supports in solo, which would once again lead to increased queue times. Potentially massive increases, depending on how many people are willing to suffer through abuse.
So that begs the question. If this was to go through, what would be the best solution?
It would be to force people, who may have absolutly NO idea how to support, to support. (and people are already complaining about having to support too much. Just imagine if you put Mid/Top and you got support) Yes this has been already done, but only for the top tiers, which have less people, and therefore less teams that can be made. Now imagine if a bronze mid main had to support, and the gripeing/flaming/feeding that would follow... shudder
Or you could just remove the communication advantage premades have via an opt-in voice chat in the client, while keeping the dynamic queue(without the 4 man premades of course, no one likes those that are not part of it). You know, like they are planning to do with the new client...
Now, which sounds more pleasant?
P.s. You didn't think you were the first to suggest this, did you?