CG has made arguments in the past that Sandbox mode:
a) Wouldn't bring much revenue for their investment, and
b) Wouldn't be used much
I think most veteran players would disagree with both of these points, with some thought as to the business model.
Before I make my points below, please don't believe that CG solved this by implementing Conquest. The Feats and the enemy / ally Modifiers don't help us learn how to fight in PvP, where it really counts for many of us. We want a "real" sandbox. One where our characters behave in their native state as though we battled other players' squads. Conquest is fun, for sure. But it isn't what we asked for.
a) One other collection + strategy game lets you set any squad in your profile that you want and change it at any time. Friends can battle those squads at any time. For free. You just have to be a friend. But since this game is a business for CG, a simple tweak to this model would be to charge players 2 crystals per attempt. I'll battle my buddy's special setup 10 times in different ways just to try stuff, and there, I've spent 20 crystals. Low overhead for me, so I'll actually use the service. With thousands of attempts daily, the dollars add up for CG.
b) Who did you ask? We'd use the heck out of it.