I dunno, in my experience it's never the winning team who turtles, it's the losing team who doesn't want to leave the base because they know they can't compete with the opposing team's superior upgrades.
Instead, I'd propose quite the opposite: give the WINNING team an advantage. There could be special rooms and areas in each map, about 3 per map, that can be "tagged" by players. Each one gives EP or credits to the team who controls it, at about 1/2 the rate of the 2min survival reward: so, about 1 EP per 4 minutes. Then the 2min survival reward could be completely removed from the game. These "secondary objective" areas would generate extra points for the team who controls them, so would motivate lots of things, like:
- map expansion (expanding/moving the base into one of these areas to better protect it)
- map control (forcing the enemy back into their base so that your team can control the sec. objectives, instead of just turtling in your base
- agressiveness (basically, discouraging turtling)
Basically, this would help prevent those looong games where one team is dominating and is inevitably going to win, yet the game drags on forever anyway. Instead, the team who has control of the game will be earning a good 3 EP or 900 creds (what's the EP-Credit exchange rate anyway?) per 4 minutes, which would help finish off the game.