Games like houdini and alice cooper are probably safer than you'd think, as they're all using P-ROC boards. More manufacturers using the boards means a better chance of replacement if any of the manufacturers dies out.
It's impossible to really speak to how reliable a board itself will be this soon. Early Bally boards are still quite reliable today, but early williams less so. Stern is probably safer than JJP as they're a bigger company. If any manufacturer's boards are going to be produced after they're gone, it'd be Stern.
Honestly, I don't think it's worth worrying about, at least for Stern. They're a large company with a lot of games out, and they're using the same boards in multiple games. Other manufacturers are still figuring out their board sets and keep changing them, or use lots of custom boards for each game, and might be a bit more dangerous down the road. Exception for games that use P-ROC like Houdini, TNA, etc. Even 40 years later most boards from the original wave of SS games when they were still figuring stuff out with sub-par electronics are still working fine; most boards now are going to be the same.