I am always up for getting new blood to the series with each title in the series. However, it should not mean that those already introduced should be left behind. It's a tough one, since there are 60+ characters that's appeared on the main roster throughout the series, and each and every one of them simply cannot appear in every single game.
So yes, that means some of our favourites won't appear in every game. It means some of them won't appear in MKX. And that's just how it is. A sensible person would accept that and move on.
I have several favourites amongst the MK characters, Jade being the #1. She's had 5 playable appearances in the many games of Mortal Kombat, those being UMK3, MKT, MKD, MKA and MK9. She's not one of the more popular characters, in fact, I remember clearly back when she was announced for MKD, the majority of the community thought she was a waste of space and would rather have Kitana instead. In MK9 she was welcomed alot more though. However, she will most likely not appear in MKX, and I am completely fine with that. As said, I have other faves, and I just hope one of them will appear in the game.
Should NONE of them appear, well, then there will be room for another new favourite character. That's how I got all these faves, because of games that lacked the faves I already had. So I welcome the new additions, because that means I can expand my gallery of favourite MK characters.
All this said though... I don't want the entire roster to be replaced with all new characters, and still be a game of the main series. Arc System Works did the RIGHT thing when creating BlazBlue, still keeping the gameplay (with some changes and adds here and there) from Guilty Gear, but made it its own game with an entirely new roster. Capcom did the WRONG thing in Street Fighter 3, as a continuation of the main series, but literally replaced the roster in its entirity, except for Ryu and Ken. They could just as well have made a completely new franchise not part of the main series.
Netherrealm Studios are doing neither of these things though. So far, NRS are doing as they have ALWAYS done, keeping some of the old characters and adding some new faces to the franchise. I think that's a good balanced way of doing things and I wouldn't want them to do it any other way.
If your favourites aren't making it to the main roster, then it is time for you to expand your mind and find a new favourite character. MKX is not all just about the roster, though still an important part. However, people tend to focus on their favourite character being in the game, and if he/she isn't, then they judge the entire game as bad, as if that one character would be the main factor as to if the game would become a success... it isn't. A good game is a good game with or without certain characters, as is a bad game a bad game. All the other factors that make a fighting game can't simply be ignored just because of one character. And if you do so, then it isn't the game, but your way of thinking that's the issue.