@Spiritus Arcane  

 
The Director´s commentary for Friendship Games says that in the first draft (the same one with Sunset Shimmer wanting to return to Equestria, a duet with Sci!Twi & an ending where the latter does not transfer to C.H.S.), also had a scene were Abbacus Cinch fell through one of Midnight Sparkle´s “rifts” into an area of the Everfree Forest, quickly transforming into “a monster” [it is not clear if it is a delayed effect caused by the rift or the “background magic” of Equestria].
 
One of the written rules of DHX for the “setting” of Equestria: “No humans allowed” (either past or present).  
-> This was set up precisely to avoid direct comparisons with G1 [since it was “inspired” from it, not a continuation].
 
(A similar one was in “discussion” for G5 in 2017: No Equestria. No Alicorns.)

portals to the other worlds

Mirror did the work along the way. The rifts were shortcuts which means either there would be no change. Or Rarity, Fluer, and etc were in for a longer, more painful transformation once they arrived.

The most powerful magic of all, plot convenience!

Well to be fair, the rifts Midnight made were less portals and more tears, so they probably didn’t come with the species switching properties the portals have.