If you read that more carefully you'll see that the letter writer isn't complaining about Beethoven or his music but that his imitators can't do it as well as he does.

He's not complaining that the burgeoning romantic movement sucked compared to the true classical music from his youth but that the quality of the music being produced has tanked because the new practitioners suck at making it: Beethoven, because he was a genius and could get it right, gave them carte blanche to break the rules that previously kept the music good.

Although it's true to say that Beethoven didn't know how to stop.