I consider myself the kind of MK fan that would buy it as long as it's the same universe. ALL characters are interchange-able to me and I'd still purchase it.
To me Mortal Kombat is a collection of stories. If you take out Sonya/Kano/Jax. The story could still continue, but a chunk of MK will be missing, but overall it's still MK. Much like taking a bite out of an apple. It's still an apple. It's just not in it's fullest form.
I think you can replace bit off pieces of this "apple" as long as it happens in stages. I'd be a bit unhappy if in one game the entire apple was eaten in one sitting, then the next game it's a brand new apple.
That's what MK is to me.
It's a combination of stories that create MK. Anybody can be dropped and it'll still be MK to me, because it's got plenty of other stories to keep it Mortal Kombat.
EDIT: Don't get me wrong.
There are plenty of characters I'd like to stick around for a VERY long time becasue I REALLY like them. but it's not the end all be all. if they are killed off.