I can understand that caz
Although I do know of a few blind, or almost dogs that are very happy (as long as you don't move the furniture around!) it may be that to reduce the risk of sending her blind during the op, the vet asks to wait until the cataract has built up, so she may need a bit of help for a little while because her vision will be reduced a lot.
However, they might be able to suggest something else, I'm not expert at all just going on things I've heard before!
Good luck xx let us know how you get on at the vet!