Presumably it makes sense to sunset PHP especially with how cheap code migrations are nowadays.
A pet peeve of mine is when people offer simple solutions that are only simple by virtue of ignoring all use cases that don't matter to the author.
Every interconnected system of concerns has simple solutions when you reduce it down and ignore the ordered effects.
And who's to say whatever sites are migrated to won't be "sunsetted" in a few decades too?
Seems like a no-brainer to maintain it.
(You need to address one specific company, because if you address two, neither will give you a cent, expecting the other to pay.)