The key point in the GP is nobody really understood how it worked anymore. No matter how accessible the domain generally might seem, once a system becomes a mysterious black box sans documentation or continuing expertise then no CIO is prepared to sign off the migration, for fear of missing a business rule or special case.

I've been the guy that left taking all the knowledge with him, years later learning that they've lost the documentation & the source code but the system is still running.

I've seen this occur in other domains (finance and education sectors) and basically the black box hangs on until either the unit, or the entire business, ceases to exist.

I did see one project that attempted a like-for-like reimplementation via a carve-out process and it was a decade-long gigadollar boondoggle that has basically crippled the enterprise that tried it.