It's worth bearing in mind that Sinclair was a general electronics company
The computer industry was just a lucrative cash cow which they milked to fund further developments, such as the QL, C5 and manufacturing processes.
Then too, they pretty much invented the "Good Enough" business model with a mix of innovative hacks and the use of cheap components (which were then often pushed past their design limits): you got 90% of the features/performance for 50% of the price, but then had to deal with functional workarounds and accept a much higher risk of systemic failure.
E.g. this article on their calculator line covers several of the hacks which they used, from repurposing a TI processor to using a plastic lens to magnify the LED display - this reduced manufacturing costs and improved battery life. And it also notes how high the failure rate was!
Unfortunately (and perhaps ironically), Moore's law worked against this business model - as the cost of components dropped, there was less of a case for using low-cost "hobbyist" quality hardware. And proprietary hardware such as the microdrive were on a road to nowhere: Sinclair simply didn't have the economies of scale to compete with the hardware used by other computer platforms (e.g. the C64 and the PC compatibles, which all used variations on the 5.25 floppy - and later, 3.5" drives), especially when government-subsidied components from Japan started to flood the market.
The world was moving on, and as the infamous spats between Clive Sinclair and Chris Curry - plus his dismissal of Alan Sugar as a common "barrow boy" - both Sinclair the man and Sinclair the company failed to move with it.
Still love my old Speccy, mind ;)