you see people don't see the drawbacks of food as far as synthesis is concerned. everything would be altered at a genetic level. everything. including food. synthesis is the final evolution of nothing. Life cannot hit a wall of non-evolution. That is not the end of evolution it is the death of everything. Synthesis has been tried before remember....ahem.....collectors. and look how that ended.
As far as transportation is concerned, we cope pretty well without intergalactic deliveries at the moment, so there is no reason to suspect that this will be a problem.
Collectors were not synthesis collectors were the Reapers creating an army to act in their stead during their 50,000 year wait between harvests. This would ensure no one suspects the Reapers exist. While still allowing them to perform tests to find the suitable species to create a new full Reaper with.
Food changing is a pretty moot point because we change as well. If there was one change without the other then maybe you might have a point.
We also don't have populations of billions with millions and millions of acres of land burn or destroyed. Injured people trying to recover from their wounds and limited medical supplies. Even if you wanted to use current day examples. Modern Continents are not fully self sufficient for certain key things.
You show such an amazing inability to see the devastation brought on by the conflict. Go replay the Palavin mission and look up at the planet when Garrus comments on it. Then stat that a planet in such condition could support a population on it's own without food shortages, fuel shortages and medical shortages.