[8note]

I'd want 5 years at the minimum?

Giant changes like that are worth capturing in long term planning processes, and then you need time to get ramped up on the new stack, design and implement the replacements, run all your backfills, and also, still have a couple years to figure out the parts that don't migrate nicely. With enough time available, you also don't have to drop actual business related improvements, even if your progress slows down a bit