duckNote has been good to me, made about 0.8 BTC off it and still own about a gazillion. That said, I don't see any CN coin as serious competition, none of the cloners understand the code and the original devs were pretty sloppy anyway from what I can see.
I really don't see many new DRK clones appearing after open source, DRK's strength is the MN network, you can't clone that. I think open source *is* going to absolutely, exponentially grow the DRK ecosystem though - ten thousand eyes spotting and fixing bugs, more new and innovative code than Evan can comfortably review alone, and a thousand startups monetising MNs both for themselves and MN ops.
I believe a bigger upcoming problem will be preventing MN bolt-ons from screwing with existing MN functionality, if only through bringing crappy underspec'd VPS machines to their knees. What's needed is a vetting and licencing system maybe for 3rd party apps?