Speaking as someone who owns a boat built in 1979 by a now-defunct company, I can safely say if we are talking small boats, it makes very little difference in parts availability as to whether the original manufacturer is in business or not. Other than the hull, rudder, hatches
and maybe portlights, everything on your boat (hardware, rigging
, sails, mast, electronics, etc.) was made by someone else other than the boat manufacturer. When you go to replace your mainsheet blocks, you wouldn't approach the manufacturer; all he would do is sell you (with a nice markup for him) a Harken
(or Schaeffer or Garhauer) set up anyway. And with inexpensive, small boats, the original equipment was often inferior quality anyway. I have found that I can buy inexpensive replacements for original hardware that is far better quality than what came with the boat.
You can even buy replacement rudders for many boats now that are superior to the originals in quality and function. They ain't cheap (check out Ida Sailor's website), but they are cheaper than a custom new unit. I think the only things that aren't easily reproducable from old boats are hatches
and porthole windows. But even then, replacements are readily available; just not as drop-in replacements.
Don't pass up a good old boat just because the manufacturer is no longer with us.