Fun Mechanic?

So instead of having players who are good constantly take the lead I have a new mechanic.

Units get stronger and stronger due to upgrading. Until they reach their cap. Perhaps 6-12 upgrades or so. This is considered maturity. Now that the XY YZ team is at their strongest, their strength starts to deplete(perhaps max hp and speed). Their units begin aging, and they must beat their rivals before they get too weak.

The only problem is what forces a player to make the final upgrade? They could just use 5 upgrades and fight forever.
hmmmmm
Perhaps finishing your Units is beneficial in some way.
Ah, you are allowed to Ally. So now the game would change a lot. The first two people to Ally would have the first start to defeat their enemies before they are ready. But as they become weaker the other teams ally, and will die later. A battle between the old and young.
Youth/Stength versus Position.