I heard a rumour that the MKIs were never properly devalued over time on the books. So, they were showing as being a much higher value than they actually were, making a phase out quite embarrassing.
Hence the refit, and I imagine a sharp depreciation on the books in the coming years.
The only reason to keep MKIs in my mind is for my own nostalgia
If they took more seats out in the ends (I know, how would we cope), it would allow for more people on board in the interim.