As much as rebuilds are fun for those inclined to dive into drafts and rosterbate, and for those who like to dream on draft picks turning into stars, rebuilds where you "tear it down" (trade away all your important players) and start over have a terrible record of producing winners unless your team both plays in LA and is called the Lakers. Cap space isn't going to lure any stars or superstars to Portland. Frustrated superstars aren't going to tell their teams to take what they can get from the Blazers because they'll refuse to re-sign anywhere else. Tearing it all down and starting over generally means being awful for a decade--so you'd better be very committed to following the draft to get your sports fun.
On the other hand, starting from where Portland is--a star in his 30s, overpaid mediocrity filling up the roster and very little in the way of exciting young talent--is also pretty unlikely to yield a title. However, winning a title takes star power so having Lillard gets you closer than not having Lillard but having a bunch of draft picks. So while neither avenue is terribly exciting or has a decent chance to pay off, I'd reluctantly rather see what (Sleepy) Joe Cronin or his successor can cook up in terms of reshaping the roster around Lillard rather than selling off Lillard for scrap and draft picks. Especially considering that Lillard will likely no longer command a massive haul. He doesn't have a ton of prime years left, he's underperforming and he was never a top-five guy like, say, Harden used to be. And even what Harden got traded for wasn't super exciting once you realize that pick swaps aren't particularly valuable.
If Portland was a free agent destination, if players wanted to come to the Blazers, if Lillard was likely to bring in a franchise-changing haul of intriguing young talent and draft picks--I'd say trade Lillard this instant. If Portland were Miami and Lillard was likely to bring in a 2019 Anthony Davis return, go for it. But as the Blazers, with 2022 Lillard--the likeliest path to relevance (likeliest, not likely) probably runs through keeping Lillard and hoping Cronin or someone else turns out to be a savvy GM who can create a stronger, nicely-fitting roster around him.