Somehow these facilities don't work on the dwarf planet but they do in the nothingness of space?
Please man. Wanting Rannoch is just greed. They could drop them on any planet and have those facilities that work in the nothingness of space provide food for them until they are picked up.
Yep, that's no fun, but this isn't about fun.
Easier said than done. Seems you're trying really hard
not to understand this. The Migrant Fleet's facilities were degrading to the breaking point even before the Reaper war. They'll go down faster without access to outside infrastructure and manufacturing. Best estimate was they had eighty years left, per Ascension. You're thinking they'll keep roaming in space afterwards, but they're looking at permanent settlement for lack of survivable alternatives, and it isn't something that can be done more than once. Carving an old hauler down to make ground structures for more people to live in is a one-way change.
And you're still assuming that the fleet will both have survived to pick them up, and be within range to do so (with the relays destroyed no less). Again, they see the writing on the wall. They're conscious of the losses their fleet will absorb in the coming fight. They can't afford to assume they'll be there to pick them up; hence they have to be able to survive permanently wherever they set down. Rannoch has the resources, in terms of plant life, atmosphere, water and repurposeable technology, for them to survive even after the tech they brought with them breaks down. "Nearest convenient rock" does not.
And do they have an endless wellspring of meds on Rannoch? No they don't.
Point being, they won't get as sick there as they would experiencing similar accidents elsewhere.
How do they eat the stuff they plant on Rannoch if they get sick from the very air there? You missed the main point. Rannoch's envrionment is just as dangerous for the quanrians as any other planet. It would still take them years to get used to it. So they can only survive those years by using their sterile liveship facilities, which they could do on any other planet. What makes Rannoch so superior in the very specific situation of ME3?
Read the codex entry on liveships. They grind down and sterilize the plant matter.
"It's the difference between sixty years and six hundred." The six hundred on another planet would be contingent on bioengineerig themselves to eliminate their dependency on their native plant life; technology which is both unavailable in the aftermath of the Reaper war and illegal in Council space.
Furthermore the whole attack wasn't the smartest move to begin with.
"Remember the sentinent killer machines that kicked our civilisation back into the time of nomads? Let's attack them with all our population at once. Let's even put guns on our civilian ships and send them right into battle. There is no conceivable way this carefully laid plan could backfire in any way. "
Their civilian ships were already armed. That's kind of necessary when you live in the Terminus systems, and neither the Geth nor the Reapers have ever distinguished between armed and unarmed ships. Where's the benefit in not being armed?
The plan deployed against the Geth was a solid one, and worked just fine until the Geth accepted the offer the Reapers made in advance of the Quarian invasion and got code upgrades which even the heretics never had.