I understand your logic, but you are forgetting something. Half of the NBA is in the luxury for 2018-19. So half of the NBA can't even buy. We have two young franchise cornerstones (Mitchell and Gobert) - a 2nd or 3rd tier player might be intrigued by joining. And since many don't have money to spend, we might get a good player for cheaper than when everybody overspends in 2019. So if we have money in 2018 (which we could), we would stand a better chance of buying a player than we would when almost all of the league has money in 2019.
When I think of possible 2018 targets, I'm thinking of guys like:
Avery Bradley
Mirotic (if his option is somehow declined)
Tyreke Evans
Kyle Anderson
Will Barton
JJ Reddick
Patrick McCaw
Ilyasova
Herzonja
And of course we have the opportunity to take a contract if we wanted. If Portland dumps a guy like Harkless and we actually wanted him, we could. This summer will be chaos since so many teams are staring down the luxury tax. Some, like Portland and Washington, will be looking at repeat offender if they don't make a move.