It looks to me like it is definitely succeeding by most reasonable measures. Its percentage of total server sales (measured in units or dollars) continues to show double-digit growth. That doesn't count the embedded market (where Linux is huge), the supercomputer market (where Linux dominates some niches) or the desktop market (where Linux is miniscule, but has some success stories).
I don't know if Linux will ever succeed on the desktop. But in the other three it has established itself and hasn't topped out yet.
Cheers,
Ben