Lillard has 3 years plus a player option left.Worth noting that this is less significant in regards to a Lillard trade than other recent superstar moves. You had to know you'd have a team good enough to compete after acquiring AD because he had one year left on his deal and could just walk if you didn't. Same with Kawhi, just to name another recent example.
This is not the case with Lillard. He has 5 years left on his deal: unless you think he's a legit threat to sit out 3+ years of a contract refusing to play (which I don't think he would) you don't need to know for sure you'd have a competitor left once you'd acquired him, you'd just need to believe you had a clear road to a competitor once you acquired him. Which wouldn't be too hard to imagine, even if it would be more difficult in practice.