You should have said it was faulty from the beginning. 😉
I think you are correct about not being able to drive an external 4K or 5K monitor. But you still might want to get one because your next Mac probably will be able to do so. Much depends on long you plan to keep the 2012 Mac.
Also consider what kind of connectors and adaptors can be used. Apple has introduced USB-C connectors in the new Macbook. We will likely see the display/thunderbolt port dropped from Macs very soon and replaced with USB-C. The USB-C connector can transport USB 3.1/3/2 signals and it will also do Thunderbolt 3 when the latest CPUs and chipsets start to arrive.......maybe this fall.
So if the very near future we could see a rMBP with only USB-C connectors doing Thunderbolt 3 towards 5K monitors and providing USB 3.1 transport toward other devices like keyboards, hard drives, printers...etc.
I would want to see how this all starts to play out with the fall new hardware announcements before putting down any serious money.