At this point (atleast for me) the issue is a combination of a few things:
1) MKDA and MKD were released back to back and, for better or worse, drastically changed up the rosters in interesting ways.
2) MKD in particular brought back fan favorites from MK3 specifically because most of them were non-existent in MK4 and MKDA.
3) Mortal Kombat is the only fight aside from DoA and to some extent Tekken where the audience actually cares about the story. They even showcased a story teaser on stage at a gameplay reveal event. Do you know how terrible Dragon Ball Z would be if the only people you saw fighting in every season were the Z-Fighters, Vegita, Nappa, and Raditz?
4) People literally asked them for post-MK3 characters since MK9's dlc and they still failed to deliver. Knowing that over 1/3 of the roster isn't from MK4, MKX, or the PS2-era is very worrying considering only 14 or less characters are gonna be split between Classics, Post-Classics, and MKX newbies.
The only way people would be more considerate is in the case of SFV (if it started with 24 characters) where you can get the DLC for free by playing the game or if they play russian-roulette and put DLC characters as NPC fights in the story mode.