As long as those manufacturing systems were air-gapped and working, I can understand being conservative with changes. Of course at some point you're at risk of hardware failures too, for things that are likely NLA.
Yeah, that's really the biggest issue with these systems. Were I one of these manufacturers I'd invest in getting the current software running in a VM specifically to make it easier to transition to new hardware. The last thing they'll want to deal with is that their software was written for Sun Spark and that no longer exists.
That said, hard to go wrong porting over the x86 at this point.
Some of the signal control infrastructure for the Melbourne rail network was ported over to an emulated PDP-11 System for this reason. This was previously discussed on HN here: https://news.ycombinator.com/item?id=42514553
My humbling Claude experience was pointing at a codebase it took me weeks to get to a good 2003 standard, and watch it do the next fifteen years, cleanup, refactoring, feature additions that needed the tech debt solved I never got round to first, in minutes.