Its been my personal experience that a huge number of companies (all types not just 'startups') tank after a while (~5 years). When I look back if I had stayed with any of my companies I would have been out of a job anyways. Therefore, I'm quick to move on when things start to have indications of going south. Loyalty way back when was probably a good means of 'survival' in that it resulted in moving up the chain into a secure job but these days I find loyalty to be the most sure fire way to being shafted.