Penny wise, pound foolish
As for the eventual ZX Spectrum itself, it would launch in April 1982 with unfinished firmware and a BASIC ROM that could charitably be described as lethargic. A plan to ship an upgraded ROM was dropped due to the popularity of the thing, with the finished firmware shipping on peripherals, ready to take over from the Spectrum’s incomplete ROM when needed.
It wasn't upgradable because Sinclair wanted to save pennies (of course) and soldered the ROM to the circuit board instead of pushing the boat out and using a DIP socket, and the ROM software was unfinished because Sinclair wanted to save pennies (again) and fell into a dispute with Nine Tiles. A lot of the functionality which was in the Interface 1 was originally meant to be in the Spectrum ROM.
Of course nowadays there's interpreters for Windows other OSes like this.