Agreed. (Congrats on your first post, btw!)
The thing is that bands don't suddenly start doing all ballads --- that would be wrong. But there's nothing wrong with a heavy band filtering their music with a little light relief, as it were. As I think I mentioned, Kamelot, one hard rockin' band, but they can put out an absolutely gorgeous ballad like "Abandoned". Nothing wrong with that.
In fact, in some ways I think it does the opposite of what you're saying --- if a heavy band suddenly brings out a ballad, more people may listen to them and begin to appreciate their music. It can be a way in for those who have never listened, nor thought of listening to this sort of music, and that surely can't be bad.
Anyway, I stand by my claim, which I've believed for over 25 years, that metal and heavy rock bands pen the greatest and most effective ballads. Yes, some pop songs are good (there's no denying the power of Harry Nilsson NOT MARIAH CAREY's "Without you" or even Phil Collins' "Separate lives" or "Against all odds"), but generally I would class them as lower quality and far less imaginative than rock songs.