I agree, this is an exploit and should be fixed. However, it's not as effective as you make it out to be.
First of all, the healer minions are the most expensive. You can only buy the first teir at the start, and if the other team gets the advantage, then by the time you get a pair of each teir, it won't help you. They'll have much more money than your team does, and if they still lose, that's their mistake.
Second, this still works on the other two generals as well, not just Sedna and QoT. I'd say an Oak with this is actually much scarier.
Third, it's not 'completely gamebreaking.' It's rarely used, and hard to pull off. A good team could probably destroy it too.