What is the origin of this alternate ending to Captain Tsubasa?
I remember looking into this a while back, but never found where the theories about this 'true ending' originally surfaced. 

The reason you see so much information about it in Spanish though is because Captain Tsubasa was one of the first Anime to ever air on TV (at least in Mexico), and it was extremely popular at the time. The theory probably surfaced there, and once it got into the internet it became an 'urban legend' of sorts.

But yeah, the alternate is purely fan-made. As far as I know the manga is still running.