You just didn't understand it... you would be able to always use only two heroes, because if you can use more (what will come to game in future), it will be too OP! So you aren't right. Let me correct it:
This is solution! I have 3 heroes and I can attack at full strength. This is completely fixes the issue... SC's game logic will stay and we will be able to attack at full strenght also when one of those 3 is upgrading.
They should add a fourth hero, and so when you are upgrading one of your 4 heroes, you can always use 2.
Was it helpful?
Here is my older suggestion about heroes: http://forum.supercell.net/showthrea...ameSketcher%29
But it's so huge and it will take very hard work, so it has a minimal chance to arrive to the game.