I already find enemies eager to target my Haste casters, and losing that concentration, especially if you have it twin casted, gives you a harsh punishment with two lost turns (that you can't negate by recasting haste). Haste is powerful, but casting haste also means you're not using any other concentration spells on that character.
It honestly doesn't seem that overpowered to me. It's on the more powerful end, sure, but there are other things on that end, and it can be countered. If anything, giving enemies some more counters (or more spells/options in general) sounds good to me. You don't even need slow. Getting knocked prone is a guaranteed loss of concentration, so giving enemies some ice surface creation can already be a good counter.
I'm also always in favor of giving enemies the same tools we have access to.
