I know they were out there. The later IBM PS/2 machines used them, and so did some Unix workstations, but the 2.8MB format -- quad or extended density -- never really took off.
It did seem to me that if the floppy companies & PC makers had actually adopted them wholesale, the floppy disk as a medium might have survived for considerably longer.
The 2.8MB drives never really took off widely, so the media remained expensive, ISTM -- and thus little software was distributed on the format, because few machines could read it.
By 1990 there was an obscure and short-lived 20MB floptical diskette format:
Then in 1994 came 100MB Zip disks, which for a while were a significant format -- I had Macs with built-in-as-standard Zip drives.
Then the 3½" super floptical drives, the Imation SuperDisk in 1997, 144MB Caleb UHD144 in early 1998 and then 150MB Sony HiFD in late 1998.
(None of these later drives could read 2.8MB diskettes, AFAIK.)
After that, writable CDs got cheap enough to catch on, and USB Flash media mostly has killed them off now.
If the 2.8 had taken off, and maybe even intermediate ~6MB and ~12MB formats -- was that feasible? -- before the 20MB ones, well, with widespread adoption, there wouldn't have been an opening for the Zip drive, and the floppy drive might have remained a significant and important medium for another decade.
I didn't realise that the Zip drive eventually got a 750MB version, presumably competing with Iomega's own 1GB Jaz drive. If floppy drives had got into that territory, could they have even fended off CDs? Rewritable CDs always were a pain. They were a one-shot medium and thus inconvenient and expensive -- write on one machine, use a few times at best, then throw away.
I liked floppies. I enjoy playing with my ancient Sinclair computers, but loading from tape cassette is just a step too far. I remember the speed and convenience when I got my first Spectrum disk drive, and I miss it. Instant loading from an SD drive just isn't the same. I don't use them on PCs any more -- I don't have a machine with a floppy drive in this country -- but for 8-bits, two drives with a meg or so of storage was plenty. I used them long after most people, if only for updating BIOSes and so on.