Cast back a bit further. There's already 12 170s brought in to the fleet which freed up both 150s and pacers, along with the extra 153s that were brought in permanently. Overall the balance isn't far off.
Also with covid services are currently nowhere near reaching full capacity levels like before do has offered some temporary breathing space.
It has also meant that no single 153s can be used, so all single 153 workings are having to be doubled up.
What happens next is anyones guess. I can see the temporary 153s staying longer, maybe as a permanent fixture to the fleet. There would be the cost and hassle of converting to PRM compliance but if they are around long enough it could offer scope for expansion of services.