A lot of variables. Huge companies can be a nightmare dealing with to get a repair done (Sony, Marantz) and will eventually let the serviceability of a component expire. Also, look at the thread on repairing Esoteric.
Small companies may close, but if they don't you're likely to get years and years of attentive care, concern, and if necessary service.
Different companies also have different identities: look at Bryston and their fabulous after-market rep.
Different components are going to have different lifetimes--a loudspeaker vs. the latest DAC. You may well want to keep one going long after ditching the other.
Also, different components have different levels of repairability regardless of whether the original company is still around; again, a loudspeaker vs. e.g. a CD-player.
This doesn't even get into issues like specialization, commitment to a product type, experience, knowledge, R&D resources, etc. etc.