This is why I don't think trying to rank Brown as top-whatever, matters. Who would you rather offer a 5-year supermax contract to, Durant or Brown? Harden or Brown? Even LeBron or Brown? How about Kawhi Leonard, you want to pay him for 5 more years? You could go as far as Beal and Mitchell also and it would be a reasonable debate that Brown is better.
Comparisons to players like Sabonis, Adebayo, or Towns are hard, they are about the same age but play different positions. But if you look at the Murray, Fox, Morant, Ingram, Lavine tier, it is still not definitive that any of these are better players than Brown. You could make the argument that Murray or Lavine are better ball handlers and might in that way be a better complement to Tatum but that is not the same as saying they will be better players over the next 5 seasons.
I do not think that this Brown contract will become a bad contract. It will seem bad to start, and then it won't.