Whether a series can continue without going stale/repetitive etc. is something you can only decide on a case-by-case basis, depending on the premise of the show, the particular writing staff involved and so on. Sometimes what is needed is a rest period where there is a ten year or something gap between seasons. Right now that is the minimum that would be needed for Friendship is Magic to continue successfully. While the quality is actually still decent, there is this sense that the franchise is going round in circles at this point, retreading old ground to the point where it has become almost a parody of itself. Take the all the meta jokes about the friendship speeches and musical numbers in Forgotten Friendship for example. While they were funny, it's also a sign that the writers are desperately struggling to find new ways of spinning this material that is refreshing.