I think people underestimate Lillard. His shine has always been taken away by Curry, but Lillard is clearly a top 10 player and hasn't shown a sign of decline yet (although that would be typical Celtics luck if that happened). But in contrast to Kemba he's an amazing shooter.
A player like Lillard is levels above Brown (or Beal or even Towns for that matter). Brown is a 'star' who can win you a game, while Dame is a 'superstar' who can win you a series. I believe such a trade would significantly increase our chances to win a championship in the short term.
However if that trajectory would fail, it is a huge loss, since it's unlikely that Lillard's contract would deliver many assets. And the wing position is very scarce. It's difficult to find wings who are adequate at both sides of the court. That's why Brown should hold tremendous trading value and why (for me) there aren't a bunch of guys I'd trade Brown for despite I feel he's overrated.
For the record, the players on the Celtics I find currently overrated and underrated:
Overrated: Pritchard (love him, but I fear he won't amount to more than a back-up), Smart, Brown, G.Williams
Rated accordingly: Brogdon, Tatum, Horford, Griffin, Kornet
Underrated: White, Hauser (very underrated), Muscala, R.Williams