> why shouldn't I be OK with buying the cheap mega-supermarket brand product
Just some random thoughts, in different contexts:
1. Buying the cheap mega-supermarket can lead to non-obvious degradations in your community (e.g. you find yourself in a news desert because the local paper went bankrupt, following the closure of the local stores who advertised there).
2. Competition keeps competitors honest, but if one company gets too much of a lead, it's competitors give up leaving fewer market-based checks on its behavior (even if it was fantastic and awesome in the first place). Eventually prices will go up or quality down, as the never-ending quest to provide increasing shareholder value continues.
3. The real costs may not be reflected in the sticker price. For instance, you pay for Facebook and Google with your data and privacy, not your money. A competitor that directly charges you $20/year may actually be a better deal.