Gonna have to agree with this, very few Mac users are going to use your application (assuming they have other options) if it looks blocky next to all their other apps. This goes for absolutely everything but especially so for creative tools.

2X displays might seem frivolous to non-Mac developers but they've been the norm on OS X for several years now.

Yeah, for me personally Pixelmator is even better than Photoshop because (besides not having an astronomical price if you don't want a subscription) they have figured out the perfect sweetspot between too much and too few features (for a prosumer).

And lately Affinity Photo which is directly trying to compete against Photoshop.

Just out of curiosity, why downvote? I can assure you that non-retina graphics on retina display looks awful. In spreadsheet software the lack of retina support may not be a dealbreaker, but GIMP is software for creating graphics.