It's not the same roster, it's having a smart grasp on the pulse of your franchise. Look, SF3 became a great game after two iterations but there is no SF fan alive that will actually argue with you if you say part of why that game didn't hit it for people was because the roster didn't grab people the same way. Alpha 3 to some people isn't as good as Alpha 2 but the roster is so packed with favorites that it's no surprise on why it is mentioned a lot more.
The biggest problem with MK is they try to do this really dumb thing where they try to tell their fanbase that they know best. You'd think after games like vanilla MK3 and MK4 they would have understood it's not a good idea to throw too much new with not enough classic because then you're alienating rather than drawing. But you still see them repeat this 20 years later in MK11 and it's mind boggling.
What MK11 is doing is taking out the characters people have built 30 years of fan love for and replacing them with underwhelming picks, newcomers that flat out suck, or guests that should not be in a fighting game to begin with. I would be surprised if Capcom ever releases another SF game that doesn't have most if not all of the SF2 roster or if there's ever a Tekken game without Yoshimitsu, Paul, Law and King. KOF will always have Team Japan, Team Fatal Fury, Iori, Ikari Warriors, they tried a game without Mai and got properly ragged on for it, but you get the gist here. All these long term fighting games have their shit figured out except MK.
Here's a standard, base, should be in every game from now until the end of time roster. We'll get the franchise faces out of the way first. The franchise faces will always be Scorpion, Sub-Zero and Raiden. Beyond that, here are those characters you put in not because "omg im repeating da tril0gy rostr" but because these are characters that have always carried their weight and help give MK its identity:
Mileena
Kitana
Reptile
Ermac
Smoke
Cyrax
Sektor
Noob Saibot
Liu Kang
Shang Tsung
There you go, 13 characters that if you get pouty over you should be bombarded with expired eggs on the street. This is a base, forever-here roster people can fuck with. From there you go to your good round outs. We're talking Johnny Cage, Jade, Kabal, Kano, Kung Lao, Jax, those types. The ones that you generally like but you could probably live without.
This gives you 19, now I personally think every single MK game should have around 27-30 total on the base roster this gives you enough slots to go experiment and go wild. Add in all the new characters or revisit 3D era ones or less cared for Trilogy cast members, but if you release those bare minimum 13 characters every game then it's something everyone that has rocked with MK can rally behind. You have the obvious fan favorite ninjas, you got your robots, you got your kung fu master and evil sorcerer dickhead, then you can add some more old guard and still have enough to fill out enough with something fresh.
MK11 itself isn't really a great fighting game compared to its predecessors as is, but I'd put up with all that janky shit a lot more if I didn't look at the character select and see there's literally only two characters I actually like to play as.