There are plenty of parts of the world where governments/aid groups have to distribute food. Most of it is staples like bread and rice. We also have these protein drink things now that brag they can replace any meal. At some point the cost of those drinks will fall to making it worth giving out meal replacement drinks to people instead of bags of rice.
I'm not sure any of those are full diet replacements though. They serve as a good breakfast or lunch, but I don't think it's recommended you replace all meals with them. At least not in more ideal circumstances.
We're hardly in ideal circumstances. Now, some of these meal replacements are true, well-rounded meal replacements that are full of nutrients. Huel comes to mind.
Yes, but where will the protein come from? My guess is bugs. It's already a staple of the Cyberpunk franchise both videogame and tabletop, where most of the world eats something called scop, or single celled organic protein. A megacorp called All Foods produces a beef substitute made from genetically modified flatworms called EEZYBEEF. Or how'd you like a pizza with vegan cheese and pepperoni made from locusts?