Fair enough. The flood would only need to get to blockade ships not beyond them. It spreads quickly. Any planet the land is lost anyways. No one can ever leave it. Wasting man power on blockade also. And let's face it, people are stupid. How long to some scientist wants to study the flood? How easily would it spread as a first the republic does not recognize it is more than a sickness but a sentient life form? How many planets would be lost? How could they keep track of every ship that left before the blockade? The forerunners lost for a reason. They treated the flood as a sickness to be quarantined not fought. Undoubtedly the republic would respond the same way. And the forerunner did this for hundreds of years before realizing their folly. Could the republic fight the flood for hundred of years? Would they win a battle of attrition if the flood infected say "Couresant"?