Its not. Its an infusion of foreign material almost like a temporary organ transplant that raises inflammatory materials in the body and can do significant harm. If the idiots want to do this they will eventually find out the hard way its not a good idea.....
It would take probably several months to concentrate enough plasma to do the job, but its probably also feasible to culture or otherwise artificially duplicate the isolated plasma components and stem cells. It could also be beneficial to enhance your own bodies concentrated plasma products with any missing factors present in younger peoples blood
Using your own components reduces the potential for side effects and may enhance the beneficial aspects .
Additionally, research also suggests that the infamatory response and activation of the immune system you mention can be beneficial and that this activation itself may actually be the mechanism providing the remodeling and regeneration process