I live in Canada, and I forsee a donor milk bank system that mirrors our (now revamped) blood bank system. Moms donate milk, and are provided with all the necessary supplies at no cost.
I'm also working towards a medical system where breastfeeding is not just supported, but seen as normal, where IBCLCs are on call for all births, where doctors inform mothers of the risks of formula feeding just the same as they speak of the risks of smoking and not using a car seat, and where direct to consumer formula marketing is banned.
Sure it is a lot to ask for, but worth working for, IMO.
Next -- blood isn't free. It costs quite a bit of money, mostly paid for by insurance. And demand is lower for blood than it could be for milk.
And while blood is handled on a donation basis, plasma and plasma products are still paid for. And the blood shortages continually prompt calls for the creation of artificial blood substitutes (blood formula!) or for going back to allowing paid blood donors.
In addition, donating milk is far more -- everything -- than donating blood. I can go down to the blood bank every 8 weeks, lie down for 15 minutes, get up, have a cookie and be done. Milk would be needed in quantities far beyond that to meet the needs of everyone who really needs formula. Yes, people donate now, but how many moms really could pump multiple times a day while nursing their own LO, caring for their LO and any other kids, etc. Its an admirable thing and I really appreciate women who do it, but I know that, for instance, I could not have done it - I don't respond well to the pump (and I had to pump 1x a day for my first year back at work, working part time), and with my first, trying to pump for a second baby would have been the straw that broke my back.
So it is a wonderful ideal to work for, but I think in the real world that ideal is awfully far away, and there are a lot of issues of race and class to be worked out in the implementation. I forsee, in our current social structure, a large danger of any donor milk system becoming hierarchical and abusive.