Plus the lifecycle on these systems may be decades not weeks. At some point it needs to be segmented physically (more cost).

In 2017 I was hired at a place to support their ERP system and regular SysAdmin work. My first month was the last patch for their 10 year old ERP system. That was key software running the entire company and it was deployed before I finished middle school. The floor was run of scanners using WinME terminals that lost config on power loss.

That business is still profitable. I did not stay long.