I hate replying right after myself, but I think this may be more serious than I originally thought:
Assuming constant fees, and fees are in BTSX (not bitUSD). At some point the delegate payrate market will become efficient, and all nodes will have approximately the same pay rate a little above the cost of the node (for example, 5%). Imagine a shock to price (e.g., a one day drop of 10% in BTC, which has happened in the past). ALL nodes now are operating at a loss, and the correct thing to do economically is to turn off enough so the remaining nodes become profitable again. That will damage the network, which may cause BTSX to drop in value, which then would cause more nodes to turn off, until a new equilibrium is reached with only a portion of the network being active.
Of course, this outcome should be avoidable by selecting delegates that promise not to do that. But how long would you operate in the red before turning it off? $10? $100? $1000 lost? At some point, delegates not being able to raise their fees may lead to a breakdown in DPOS.
EDIT:
Not allowing delegates to raise their fees is the same policy as China not allowing short selling in the market. It sounds like a good idea, but leads to crazy economic outcomes. Even China had to give up on that idea:
http://www.bloomberg.com/news/2013-02-23/china-to-expand-short-selling-program-as-part-of-reform.html