Mortal Online just cut a tonne of staff and didn't they say they needed 1500 more subs to break even. Darkfall continues to make changes and add new things at least to keep people subbing off and on. It is the rate of change that matters as well.
Each game is in a different position. I don't think Xsyon will fail - but it could become less and less successful which means less subs. . .equals less money . . equals slower development. . equals less subs. . etc etc.