DFP is basically AdSense and AdSense also tends to go down each month. It is not easy to live from ad revenue in these times with mobile majority, ad-blockers and price sensitive advertisers...
I use DFP and have been very happy with the results. I use it for direct paid ads and back fill with adsence.
I also use adsense directly. The biggest advantage to using DFP and Adsense is that you can show more than 3 ads of the same type but I have recently changed my strategy a bit on that. I currently have 4 slots with DFP. 1 header, 2 sidebar, 1 footer. I then use adsense responsive text ads 1 header, 1 after first post, 1 footer and 1 300x250 in another location.
Guest see the most ads (they are the money makers ), regular members see a few less, donating members do see any ads. This strategy has worked very well for my but has taken a few years of fine tuning.