The problem with mobility over firepower and armor still remains: most of the maps are made such that its just impossible to flank early or abuse view range advantage. Damage and especially kills mean easily the most early in the battle, and if you cannot be active you cannot contribute. Outside of certain few maps the strengths of many mediums only start working when the battle is already developed, enemy has lost some tanks and at its worst one side has already gained the upper hand and will probably be just mopping up the other team. Just look at the WN8 expected values for Leopard I, and that one is a tank few newbies will grind(T10 after a very unforgiving medium tank grind) so it supposedly is played by a bit better players or in CWs.
It doesnt help that for mediums most maps only have 1 or 2 best spots and usually more than 1 tank in those spots doesnt help. Like the center ridge in Fisherman's Bay: if the tanks there can stay alive and not die to arty its hardly worth it to have more than 2 or 3 mediums there(1 overviewing the Eastern map and 2 others the city side). To not be useless until the battle is won or lost already the others would need to go to places where mobility is hardly vital.
That said, I still very much prefer mediums because they're not only fun, but can be actively of use through the battle in different ways. The current metagame and the playerbase favor big guns and camping, so having one brave man attempting to grab the initiative from the enemy can help the team a lot. I can solo 61-65% WR at tier 6 and 7(less at tier 8, (my) T8 meds arent very good
) and have great time doing that.