In 2004 I picked up my Warsensor WS-66. The gun itself is the definition of old school. The feedneck is the same as what came on the VM68. The internals are similar to the VM and Spyder markers. The gun weights 10-15lbs depending on what is on it. It's big, it's heavy, and it looks just like an M-4.
To me, that's what I wanted. I like that "look" just like others may like rasta anno or custom milling. For me, that's what "spoke" to me. It felt right when I held it and while it did have it's own limitations I didn't want to play with anything else. At one point in time, I had three of them. The one I have today is still my original one.
While my Warsensor has a magazine, it is purely for looks. We would have liked to have it be a working mag but, in 2004, it was the closest we had.
Today, MilSim and MagFed are hand in hand and, I feel, there are three driving factors as to why you've seen an explosion of MilSim and, in particular, MagFed markers.
First, is what I call the "Call of Duty" factor. CoD and Battlefield video game series have brought modern weaponry to a new audience of potential paintball players. Now when a player gets into paintball he may want to have his favorite gun from CoD or Battlefield when he walks onto the paintball field.
Second is the Airsoft bleed over. Airsoft is a lot less intimidating to new players than paintball. The hits don't hurt as much, it's lighter to be fully geared out, while initial cost may be slightly more long term cost is less, and the community of airsoft is a lot less judgmental when you come dressed out in full tactical gear. The gripe about airsoft though is that if someone refuses to call their hits, there is no "sign" that the player has been hit. Also, the "fear" factor of getting hit isn't the same as paintball and some players find the sounds and "pain factor" of paintball to be more alluring to Airsoft. While paintball has its share of wipers, the fact that the round makes a physical mark on the player also makes it harder to cheat and this as well is more alluring to some who first took up Airsoft.
Lastly, and this goes specifically for MagFed, the idea of a limited paint game can make the cost of playing paintball substantially cheaper. It also brings more movement back to the game. Lanes are no longer blocked off by ropes of paint when it comes to MagFed games. Players have a better chance of moving while other players are changing mags. This gives magfed games a feel of early stock class pump games where accuracy mattered more and paint loads were lighter.
Now with the BoxRotor and Dye DAM combination, you may see a new layout for paintball markers in general. It has a 320 round capacity and, without the hopper above the marker, gives a much lower and tighter profile than a traditional hopper. I would not be surprised to see more speedball style markers use a "magfed" layout with similar feed mechanisms.