I originally wrote "a giant headache that never goes away", but changed it because it is possible to renumber the networks. But... that doesn't properly fix the problem. You'll hit the exact same issue the next time you go through a merger.
Worse, RFC1918 isn't actually that big. There are plenty of companies out there that have either exhausted it or have to be very very careful with their use of it to avoid running out. At some point you run out of space to renumber into. You're also going to have issues with e.g. VPNs to people's home networks, where renumbering isn't viable.
I originally wrote "a giant headache that never goes away", but changed it because it is possible to renumber the networks. But... that doesn't properly fix the problem. You'll hit the exact same issue the next time you go through a merger.
Worse, RFC1918 isn't actually that big. There are plenty of companies out there that have either exhausted it or have to be very very careful with their use of it to avoid running out. At some point you run out of space to renumber into. You're also going to have issues with e.g. VPNs to people's home networks, where renumbering isn't viable.