What was the last successful live-action Marvel series?
they don't have a lot in the first place:
Blade was on Spike 2006, but it was 2 years after the last Blade movie which mostly sucked and lost steam.
before that was Mutant X and Night Man, which weren't even Marvel comics, in 2001 and 1999 respectively.
then The Incredible Hulk in 1977, which was... incredibly successful.
so 1 out of 4 may sound bad but that's light compared to how many non-successes or failures a "traditional" studio has.
this being the first live action show Marvel put out since acquired by Disney, who also owns the network it's playing on, i think all things are looking positive.
(oddly enough, DC comics have a lot more successful runs on TV with Wonder Woman, Superboy, Swamp Thing, Lois and Clark, Smallville, Birds of Prey and currently Arrow over the same time frame.)