They are still killing it in the +7 ratings.
The DVR numbers are a decent gauge of a show's overall popularity, but ABC still makes the vast majority of their money from TV advertisers. People who watch on DVR almost always fast forward through commercials. Advertisers know this, and thus will not pay more for advertising time based on DVR viewers.
The Nielsen ratings are flawed, and as time goes forward will be less and less important. But at this very moment they're still the best indicator of a show's chances.
If ABC is happy from the ad revenue that SHIELD is generating, they'll renew it. If it isn't, they won't. Other factors come will into play (DVR, streaming, OnDemand, DVD/Blu Ray sales, network politics [SHIELD will benefit from being owned by the same company that owns ABC], tie ins with the rest of the MCU, actor contracts, creative staff contracts, etc) but the most important one is the Nielsen C rating (we don't get to see those numbers generally, but the L+SD numbers are usually held to be a better analog for the final C ratings than the L+3 or L+7).
I'm generally a pessimist and not actually a fan of the show, but if I were betting I'd lean towards it getting renewed. Another factor that works in its favor is its episode count; shows that can get a certain number of episodes under their belt can be sold into syndication. At the end of this season SHIELD will be at 44 episodes, which is 1.5-2 seasons away from common syndication numbers. If it does get a season three, season four is all but guaranteed for that reason.
