I am reserving judgement on Simons. It is a problem in the playoffs if a core rotation player is weak defensively. But that doesn't mean he can't help the team. As to Lillard in place of Simons or in addition to Simons, I think one or the other and I would rather have Lillard for a couple of seasons at $6M per than Simons at 4 years and $125M or whatever he might get (that is about what Poole and Herro got). I can't see having both. If we are able to sign Lillard, then it makes it an easier decision to trade Simons (at the deadline).
But I am not ready to rule out either Lillard or Simons due to defense. Curry is not exactly an all defensive player. Neither is Mitchell or Brunson. There can be a place for players like this on very good teams, up to a point. If Sam Hauser can play sufficient defense, I see no reason that Simons can't (admittedly on smaller players).