Note: The following assumes season 4 will be the last season, if not, apply to last season of the show.
The thing is, Hasbro wants as many "character redesigns" as possible. If season 4 is the last, and the season has no new characters, and no new character designs, Hasbro will not see much additional revenue. However, if these are introduced, they could have several special editions that will most likely sell a good amount of units. Basically, adding as many new characters and designs into the season will lead to extra money to Hasbro. The only negative would be the loss of future sales due to fans not liking the changes or the monetization of the show, however, since the show would be over, that wouldn’t make any difference. When the series finale comes around, Hasbro has every reason to “ask” or “suggest” for redesigns and therefore new toys to sell. It is possible for the writers to object to Hasbro, but, since they are quite literally on Hasbro's payroll, it doesn't seem likely.
Basically, Hasbro has good reasons to want the rest of the elements to become alicorns, in short: money.