Here is the lesson I learn from Team-Builder, which I can infer on your suggestion.
Some players would benefit from your suggestion much more than others. Assassin players. Rengar players. Maybe some Sona players who miss "aura sona". They all would very much like a time machine server that lets them play their favorite characters when these characters were at their strong points.
ADC and support players would benefit from this suggestion a lot less. ADCs are much less incentivized to play in a dfg-W-Q-dead environment, or VS 4 ferocity rengar, etc. To some extend Supports are less incentivized to play in an environment where close ADC protection is the above all thing to do. However it many supports who want to protect an ADC are disincentived due to the ability of both teams to just trade ADC deaths and fight 4v4
Some of the earlier historic play modes also feature low farm junglers ("second support jungler") which is also unpopular .
The end result of this is that the ADC and Support roles choose to play on the season 7 server, and the season 4 server is populated by Rengar, Fizz and Riven players to exclusion. Which is exactly what happened with team builder as supports and adcs (in season 3-4 nobody wanted to play adc, even less than support) chose to stay on ranked for the better matchmaking, leading to mid and top players who joined TB with the hope of "getting my role" with long wait time in and out of lobby and no way (or desire) to complete the team composition themselves.
A few months of this and team builder became a ghost town.
YORICK MUST DIG THEM A GRAVE
