The One Flaw
The Sinclair QL was an amazing computer for the price.
However, I live in North America. I did pick up a brochure for one at a computer store, but I don't know if they actually brought any in. I do know that I never saw any Microdrive cassettes on sale anywhere, but then perhaps they just had to be ordered direct from Sinclair.
And, instead of a hard disk, it was to offer a module based on wafer-scale technology - which did not materialize.
Had it offered a floppy disk drive, it would have been more expensive, but it would have still been cheaper than the competition, and it would have had a real chance to succeed.
However, there's no need to turn the Sinclair QL into an Atari ST, since we already had - several years later - the Atari ST. And that, despite having a successful run, failed too. More ignominiously than the QL, given that the QL, like the Amiga, still lives on today among retrocomputing enthusiasts. (No doubt the Atari ST does too, but to a lesser extent.)
Apple was one of the partners in developing the PowerPC, and the Amiga, the Atari ST, and the Sinclair QL together weren't successful in creating a market that would lead to 68020-compatible processors, with improvements and advancements, being available right up to the present day in the same way that 8086-compatible and 80386-compatible processors, with improvements and advancements, such as x86-64, AVX-512, and so on, continue to be available to this very day.
That's "failure". It may not be a very fair standard, but in practical terms, it's the one that counts. Offices everywhere aren't, at least a sizable fraction of them, browsing the Internet, drafting letters on word processors, or calculating with spreadsheets on descendants of the Sinclair QL, they're all using descendants of the IBM PC or maybe the Macintosh. Even those who are using those descendants of the IBM PC to run Linux instead of Windows.