He has been playing for the team for a while, and yet they haven't accomplished their goal. It's understandable that Lillard would make a decision to join a team that he believes, with his help, has a chance to win the NBA Finals. Just imagine if Lillard had already been on the Heat roster in the last NBA Finals. I'm sure they would have had a big shot at winning.
He surely can not wait another season for the team to progress, but if the Portland Trail Blazers have proven that they have produced a team that can likely go into the play-offs maybe Damian Lillard could get back in the window of open trade, back what like what the Golden State Warriors did to Gary Payton II, but surely that is for Damian Lillard to decide if he still wants to return back to the Portland Trailblazer, and because his value is just huge and not like Gary Payton that doesn't have that much amount,
I don't think he would change his mind if that were to happen. He has already spent over 10 years with the team, and I'm sure that's a significant amount of time to figure out how to win. The issue may lie more with the management rather than the players or coaches, as they have struggled to build a championship team. Therefore, it's understandable for him to move on to the next chapter of his life with another team.