Unlimited unit selection and MBS would radically change the game. It's not something I'm pulling out of my ass, it's the truth. You're adding/modifying mechanics that weren't in the original that would completely change the way the game is played, creating new strategies and rendering others completely useless. It's not a good thing to do for a game that is 18 years old and is played by people who know everything there is to know about it and still come up with interesting tactics. It's just like if you changed chess so you can move all your pieces at once. MBS would allow players to create units en masse without having to manage their base at all except for building additional structures or assigning building to groups. You'd see a swift switch to micro management instead of the great balance between micro and macro that's in BW and that makes it so exciting.
Unlimited unit selection would make moments of panic much rarer and less prone to mistakes. This is when you see if a player is really good or not. Being able to control only 12 units per group has a huge impact when you're caught off-guard and have to retreat or move to another position. You have to move all your units the best you can to avoid heavy casualties. With unlimited selection you can just 1move and be done with it. Marine & Medic armies would move far more efficiently with unlimited selection and you would also be able to stim all your Marines at once instead of having to actually select them group by group and then stimming. Zerg players who play with Lurkers sometimes count on the Terran player to mess up moving their units out of the spines range. Same goes for Reaver drops into worker lines. You often have more than 12 workers in a base. Select them all and move them away and the Reaver attack is far less threatening all of a sudden. Psi Storms? Just move your whole army away from them instead of having to micro the units actually caught in the storm, which requires effort. While you're not micromanaging the rest of your army, the opponent could land another storm and you wouldn't have time to react. Storms behave rather differently in SC2 and are much faster to compensate for the unlimited selection. And also, like you said, MBS + unlimited selection would make Zerglings unstoppable unless there are massive balance changes, which would further change the game.
Also deathballs in themselves are not the problem. There are deathballs in BW as well. The crucial difference between the two however is that you can't move your deathball as one entity in BW while you can in SC2.