Take a look at the original trailer for Dark Sector, released long before the Xbox 360 was even announced. It was the first ever trailer for a game of this gen (what we called next-gen back then), and it was pretty amazing.
Now I'm sure you remember it if you were around back then. But notice any similarities? The Solar Rails, the sleek techno-suit, the Grineer? This is Warframe!
Even the two enemies shown in that trailer are in Warframe. Warframe is VERY clearly based on the original Dark Sector concept. It uses the same aesthetic, I'm sure some of the models are straight from it. It has the same story, and the same enemies, even the Powers system is in there. And the same names for things. Look at the end of the trailer--Hayden gets in the ship just like the Warframes do at the end of each mission!
I don't think it's cheap or anything, I actually think it's pretty amazing. I always wanted to experience the original Dark Sector, and it's great that they found a use for all of those assets that just got thrown away. I also think it's cool that a game that was totally single-player was re-purposed into multi-player, with so many of the same concepts being used. However, I do wish Digital Extremes would talk about this--it would be really cool to know the genesis of this, if this was something they were working on the whole time, etc., what their plan has been and how it has changed. Would love to get in touch with some of those guys.
Also add me in the game. Katori.