Very interesting...
to tell the truth, all those organic menaces in Sci-Fi are slowly begining to get on my nerves.. Zuul, Zerg, Flood, Alien, various organic ships/parasites, etc.. In hte end they never make sense since technology pwns organic evolition. No matter what you come up with, something built by technology will be more durable, more deadly, more easily replaced.
b.t.w. - how does the flood assimilate technology?
Aside from the fact that you're wrong, the Flood has all the best assets of an organic
and a technological menace. When it infects someone, it gains their memories and the skills they possessed. The Flood are not only a biological menace, but they can use ships, weapons, and everything else that ordinary 'technological' civilization can.
They have everything we do, and more.
You're also making the assumption that the 'initial Flood outbreak' was localized, when, in fact, all the evidence suggests the Flood was of extragalactic origin. For all we know, the Flood first arrived in an enormous number of ships, descending on hundreds of Forerunner worlds at once.
If
I were the Flood, that's how I'd do it.