Honestly, an RS-1 is still a really nice 2D image. I don't think there's going to be much <$3k new that's going to noticeably improve upon it (or even match it in some cases). The only real negative with them is their appetite for expensive bulbs fairly often.
Keep in mind that was a ~$4.5-5k projector that's ~5-6 yrs old, so most sub $3k projectors will be a downgrade for 2D IMO.
I don't really get the 4k hype, it's going to cost an arm and a leg to be an early adopter and the content is extremely rare now.
If you want 3D then you'll have to upgrade, so I'd say that's the call you need to make.