What ultimately happens to mining if blocks contain mostly non-Bitcoin data? I'm trying to understand what can happen if 


A block consists entirely of 3rd party (xCoin) data like Namecoin  
There are no Tx fees to collect since they are on alternate chains.  
Assume that the Block reward is quite low (0.25 BTC).


How will mining be profitable?  

If the 3rd party data must subsidize mining, what would that subsidy look like within a block?
Whoever discovers the block can choose what information to put into it. Therefore a miner that finds a hash that has a sufficient level of difficulty can choose to only include transactions that actually pay their transaction fee.