When I was young & foolish ( I'm old & foolish now ), I used to think there was a " conspiracy " amongst garages, to repair one thing & " fix " something else, in order to ensure my repeat business in the near future.
Finally figured it out......it's called " shxxx happens". They make bumperstickers to confirm this!
It is quite common, to repair the obvious & overlooking the cause.
Case in point, you may have an alternator problem, replace it, & 6 month later need a new battery. So what. That's just the way the cookie crumbles.
I would not worry about this guy a whole lot.