I definitely agree with giving a boost in stamina through either
a) Gym
b) Training court
Altough I'd say it should be something like a maximum of 1 pops a season with level 3 of any of those and a little bit of added cost, I don't agree paying extra 15k on stamina, 5k should suffice
That way it would balance itself with the decrease in stamina for coaches who do not train stamina and it wouldn't give a massive advantage. Those who'd like higher levels of stamin (10+) would still need dedicate themselves to training only stamina.
I very much think that stamina training should be incorporated in somewhere else, if it is not already incorporated (sometimes I have my players popping up stamina randomly)