If you really are concerned, I would suggest you focus future bike buys less on the company you buy from, and more on the components used to build the bike.
Bikes assembled using quality, generic, non proprietary parts will be serviceable for years - even if the company that sold it to you folds.
This concept brings bikes sold by the big 4, and what they get for them, into question. Assume for just a second something happens to one of those big 4, the one you bought YOUR bike from.
Are ALL of the parts required to service your bike still available with that company no longer able to supply them? No? What's that make that expensive big 4 bike worth now?