They've always been for the money. Its why there were comic books, sound tracks, action figures, and a live tour for the original arcade game. Back in the day, great gameplay could add to your money, but now, in the US at least, it doesn't really make that huge of a difference.
On top of that, there were numerous updates to the arcade that would often have significant impacts on gameplay. That can't really happen with the console versions since they have netplay. If things changed too much an updated copy wouldn't be able to play against a non-updated copy.
And also, the 2D MK games had a fairly simple engine that was reused and tweaked over the years to produce UMK3. They didn't do any real reinventing, but you see subtle changes that make a major difference.
I've been programming my own game engine recently (or at least trying too) and I have been studying the MKs to do it. Its pretty funny how things work out. Even things like character animation timing. In MK1, the character image changed at the same rate as the rest of the screen, but that led to times when you would punch and the punch would overlap your opponent for a frame before any reaction occured and that just looked sluggish. They tweaked it over time until in UMK3, the character animation is 1 frame behind everything else on the screen, even the shadow. The shadow is 1 frame ahead of the actual character. By doing this, things appear subtly different, and things seem to happen much more crisply, and the fact that the character is a frame behind his or her shadow or background animations doesn't really make a difference.
Then you add in Damage protection, something sorely missed in MK2, then in UMK3 take away the abuse of the corner roundhouse by adding pushback, plus having the moves disable the way they do based on both timing and number of comboed hits...
The original MKs evolved A Lot over the different releases. That just isn't happening with the new ones, and why should it? Back then, there wasn't much more they could do to make the game better since the hardware was pretty much the same running it. Now though, they can add in tons of hours of one player gameplay, vaults full of goodies, and all they have to worry about is not filling up the disc.
