Welcome to the harsh realities of life as an also ran.This happened to us first when Portsmouth took James, Distin and Cole from us, effectively the spine of our team at the time. Then we sold Barton to Newcastle and SWP to Chelsea. No team outside the elite can build a strong side anymore because as you rightly say as soon as they unearth a gem the top teams cherry pick them. Do I like it or think it's right? No I don't but it's been like this for at least twenty years or more. The foolish clubs voting for ffp to try and stop the elite getting stronger just shot themselves in the foot and condemned themselves to the wilderness forever. Idiotic.
As for Grealish you should be cheering him not booing him. The lad is Villa through and through. He not only stayed with you at least two years longer than he should, to get you back in the premier league and ensure you stayed there, he then signed a new contract with a ridiculously high release clause. He and Villa knew he would move on and he could have run down his contract like Millner did to us and fuck off for free. Instead he got you 100 million to rebuild your team. Time will tell if he was a big fish in a small pond but he contributed far more to Villa than I fear he ever will for us. Personally I'd take 80 million and Watkins from you and you could have him back in January.