Ask HN: Should a established biz consider applying for YC?

I've been asked by a few colleagues who are still slightly bouncing around in their Startups if they should apply for YC, Techstars, aka incubator. A few have healthy startup revenues, they have offshore "staff" but have run their biz like "lifestyle businesses". The words they keep telling me is "I want to create a sustainable model". Would you advise them to apply or should they go another route?