Rather than remove Retrieve mode, why not just change it? I've sent in my theory on changing the weights of the different modes, but I don't think Retrieve mode is what it should be. You're not really retrieving anything. It's modified, speed based defense.
I would say they need to modify Retrieve period. Make it similar to Infiltration, but a little more advanced. Instead of catching the enemy and killing them for cargo... why not have cargo spawn in from a position at the end of the map? Sneak through the enemy lines, or else brute-force your way through, in order to get to that cargo, then drag it back out before the time limit. It'd still be a fast-paced game, and probably even more so, but I think it would fit the idea of a retrieval better, as well as making the mode more difficult.