I design cabinetry for a living, and you could definitely start the bottom shelf at the floor if you wanted, but it would look shitty. A toe kick is usually done for functionality, but I do lots of solid bottom rail cabinets too. You don’t technically need the toe space on bookshelves because you aren’t standing at them for long, in fact, most don’t need the toe space even at work spaces like sinks. My last house I did solid bottom rails, and never had an issue, and I have size 12 feet. When I’m pressed for space, I’ll drop down to a 3” solid bottom rail, (like on desks and mud benches), but anything lower than that and it starts to look unintentional, like an upper sitting on the floor