This is just speculation and they way i think it will happen, and no its not based on actual facts because there aren't any.
It will be released on pc, once gtaiv for both 360 and ps3 have reached a certain stage in their sales revenue. As much as rockstar wouldn't want it to be released on pc because of piracy and modding issues that is not much of a concern when it comes to getting paid. Once they feel they have reached an adequate level of sales (sales revenue) and see a steady decline, they will port it over to the PC to double up on their revenues while the demand still exists for their product (before it looses its appeal).
At this point most of you have probably read reviews (ign, teamxbox..etc) this game is fucking huge, only word to describe is it revolutionary cause from this point onwards games will not be the same.
Personally i think they will ship a modified version of GTAIV and its multiplayer for the pc. If they were smart and interested in banking as much money as possible (which is the objective of any business especially after it has the perfect product) they would increase the player limit from 16 to atleast the conventional 32-64. Now everyone that had it for their next-gen consoles-if they cant get enough of that multiplayer will go invest in pc's that will run that game and buy it for the 2nd time
They would be stupid not to port it to the pc with a multiplayer, and i reckon the PC version will be even better cause they will watch and get feedback from the versions released on consoles to make improvements (if needed) and they will have more time to develop a bulletproof MP system for the pc. Obviously, this is from my perspective from where im standing and it may make sense to me and you but rockstar may have a different game plan..either way rockstar will not dissapoint its customers, just look how incredible gtaiv is.
Long post ^^ and i still didn't address the topic of this thread.
Will sa-mp fade, well i think sa-mp has almost reached its peak and if not maybe max at 10000players...and after gtaiv is released some of those players will leave for greener pastures but a majority of those players will still be here. You cannot come to any conclusions as of now cause it go any direction atm maybe 0.2.5 will be that so good that it keeps people around and if its not what they expected then they might leave..its too early to tell.