The first hard-drive in a system was made C: to reserve A: and B: in part because there was software out there that assumed A: and B: were floppy drives and could cause problems if something else was allocated to those signifiers. There are many things that are due to long forgotten compatibility issues like this (try naming file LPT1 under Windows to see another). Another reason is that the BIOS on many PCs was just hard-wired to assume two floppy drives so DOS would see that even if there were no drives really there.
IBM PCs, even into the PS/2 days, would boot into Cassette BASIC if you didn't have a floppy drive or boot disk inserted: https://en.wikipedia.org/wiki/IBM_BASIC
My high school had the PS/2 Model 25 SX, introduced in 1992, without hard drives. All of the software ran off of a Novell NetWare file server, but required a bootable floppy disk. If you removed the floppy disk, they would boot into the built-in BASIC from the machine's ROM.
They were relatively rare, but there absolutely were DOS-era machines without a floppy drive – found in schools, corporations, government agencies.
The most common implementation used Novell NetWare's RPL protocol to boot using an Ethernet card with an installed RPL BOOT ROM. The machine didn't need any disk drives (hard disk or floppy) at all, it loaded MS-DOS off a NetWare server, which was also used for all files or data.
You could also install a hard disk for local storage, without installing a floppy drive, but the more common configuration was no disks at all.
Also, the original IBM PC supported external floppy drives; as the 80s went on, external floppy connectors became increasingly rare, but if you specifically wanted an ISA FDC card with an external connector, they were still making new ones into the 1990s. (These didn't use anything like USB; they took the signals of the internal floppy ribbon cable, and surfaced them on a port on the rear of the card.) So, you could have a machine with a hard disk but without a floppy most of the time, and you'd plug in an external drive when you needed it, and you could keep it locked in a cupboard out of the hands of the plebian users the rest of the time. I doubt many people did this – diskless workstations were a lot less work – but probably somebody did at some point.
There was a lot of experimentation back in those days. The Wikipedia’s page on floppy disks is surprisingly long!
IBM PCs have never supported 8" floppy drives, but it was easy to make an adapter between IBM PC floppy cables and 8" floppy drives, so there have existed IBM PC clones from countries where 5¼" floppies were scarce (e.g. Eastern Europe), which supported the attachment of 8" floppy drives.
The original 5¼" floppies had only the size advantage, but they had both a lower capacity and a lower speed than 8" floppies, so attaching 8" floppy drives would have been a higher performance option in the beginning.
Only after the IBM PC/AT introduced the high-density 1.2 Mbyte floppy disks, the 5¼" format matched (actually very slightly exceeded) the performance of the old 8" floppies.
But in the context of drive lettering, although CP/M supported 8" floppies, MS-DOS never did. MS-DOS adopted the naming scheme, but not the boot style.
Even if you only had one floppy, it was both A and B, so you could say "copy a:DOC.TXT b:" and it would read the doc and then ask you to insert the floppy you're considering B (and depending on file size and available memory, sometimes switch back and forth a few more times)