Beyond this, Spain altogether lacked any interest in keeping its possessions in Africa.
Mind, note that Spanish Sahara and Equatorial Guinea both included extensive territory on the African continent. No one did that, not even (say) France in Gabon or Portugal in Cabinda. Spain doing that would ensure political complications, not least with Morocco.
Western Sahara receives something like $800 million per year in subsidies from Morocco, according to the 2011 United States diplomatic cable leaks. That's a lot of money for a region with a population of only around 600,000 people and with a GDP per capita of only around $2,500. In fact, that means that the subsidy is well over half the total economy, which makes it one of the largest subsidy programs in history (source).
Even if petroleum deposits that are believed to be in the region were discovered and developed it is thought that the area would likely still be economically nonviable. The region doesn't even have much water outside of the occasional oasis, and the primary economic activity is goat herding by nomadic peoples.
Spain would probably subsidize it more extensively still, given that it is a prosperous high-income country.
At least superficially, I suppose that there is a case for imagining Spain to keep a part of northwest Africa. It kept Ceuta and Melilla, after all. The problem is that Spain seems to have distinguished between those old sovereign territories and the newer Moroccan colonies. More, Spain did withdraw in the past; it let northern Morocco unify with the French protectorate, and it withdrew from Ifni.
I do have the feeling that you would need to change the Spanish transition somehow. I do think it possible: Unlike with Portugal, where the colonial empire was the direct cause of the revolution, Spain's colonies were relatively peripheral. A Spain that was willing to run the risk of tensions with Morocco might have done it. The only question is how you get that done.
