The problem with this theory is that all of the “somebody elses” were also there at the time, so if “somebody else” was going to do it, why didn’t they?
Well, they kind of did. We don't think much about Commodore these days because it died in the early 1990s (mostly from competition with IBM PC and compatibles, not so much Apple), but the Commodore 64 was the best-selling computer of the 1980s. And in Britain (and in many parts of Continental Europe) Sinclair and Amstrad were huge but people from the US hardly recognize the names.
Commodore is probably the closest analog to Apple because they tried to follow up the 64 with the Amiga. It didn’t work out for them (hell, it barely worked out for Apple) but some people absolutely loved the Amiga.
Do you think in a world without Apple, the founders of these companies would have ended up doing what Steve Jobs ended up doing? If so, what stopped them?