Fully agree that if food is included, and the quality is not up to snuff, complaints are justified.
While debundling food from the cost of a sleeper ticket would remove some of the gripe, reasonable quality food options are still a basic necessity on longer trips regardless of how they're paid for. If the food is bad enough that I won't eat it regardless of the price, I won't take the train if the trip is long enough that I need on board food. Admittedly, the couple times I've had the flex meals I've found them to be okay if not amazing, but it's been a couple years and on at least one of them I had the option for the cold charcuterie tray, which at least had relatively fresh elements to it.
Personally, knowing the history of bunding food into the sleeper fare (basically ensuring there's enough demand to justify food service beyond a cafe car,) I don't think debundling it will cause food service to get significantly better - if anything, it seems more likely that we'll be stuck with cafe car food alone. I'd much rather focus on making sure the food that's served is high-quality than to try and remove included meals from sleeper tickets.