Is the hypothesis of the scientist I read in his article, an accurate prediction of a major downside from using spike protein mRNA vaccines?

This is taking a possibility and extrapolating it to its worst possible outcome. In actuality, the production of the spike protein will be transient and so this scenario he fears cannot happen. There will be other consequences in susceptible individuals to trigger an autoimmunity based on proximity of the transient spike protein, but not requiring its persistence to be a source of aggravation, that will be more insidious and a longer lived dysregulation of things. But as we told you, it will be minor in consequence for the most part, and will be a low percentage of individuals who suffer. This does not outweigh the benefit of vaccination for a significant number of people who would be severely harmed, even if they recover from the acute illness itself, because there will be organ damage they undergo and this will set them up for problems later on. Not to mention the post-covid syndrome becoming an issue for a significant percentage of infected individuals. These victims, too, will be plagued with a long series of consequences that will end up being life shortening. So this is a kind of exaggerated fear he is propounding and is being impulsed to do so by the dark Extraterrestrial Alliance as part of the disinformation campaign to sow fear and confusion, and can be disregarded.