That's the long lived edge case – I didn't really go into detail, but what the heck.
AIUI, assigning a new IP address to a mainframe isn't really hard: I'm told by reliable sources it's "just JCL, like everything else".
But assigning an IP address to a machine that's had the same one since the mid-eighties? When it has machines talking to it that don't have the concept of "DNS resolution", running programs whose source code was lost a decade ago? And all those things are the revenue critical systems? That's hard.
Talking with colleagues that have done large IP range sales before, this is pretty much the dominant strategy – your agreement sells all your contiguous space, then you have an immediate lease back provision for carved out space that gives you ~decade(s) of lease time to unwind whatever business processes exist that require those legacy IPs remain around.