They got 4 unprotected picks and 4 unprotected pick swaps. Plenty of reasons to believe that is a better long term package than Brown and Smart and a couple of almost certainly mid to late 20's draft picks.
Not sure I buy that. Brown alone might be better than almost all those picks. He is playing at an All-NBA and All-Defense level at the moment and could be at that level for many years to come. At minimum, he certainly will be an All-Star level perenially.
He would have been easily the best player moved in these mega-trades that occurred this season in exchange for the mega-star. Much like Simmons and Philly, there would have been zero need to include a boatload of picks because of the quality of the player going back to Houston.
But they wanted more than Brown. They wanted an All-Defense 1st team defender too.
Now maybe Houston preferred restocking their draft capital over adding, compete now players like Simmons, Brown and Smart. Or maybe Morey and Ainge thought the price to high for Harden given he could jet after the 2021-22 season and pulled out.
But either way, I think it safe that at least two NBA GMs felt their extremely young star player(s) were as valuable or more valuable than those picks.