Will ASIC miners hold the network "hostage" and demand a fee for their services?
The 'race to the bottom' is still continuing in the mining market and hopefully will continue. If any of the miners tweaks transcation inclusion like you suggest that would make them less competitive in this market.

The miners have pretty much no power to enforce transcation fees while the market is functioning as intended, so your question is somewhat moot. 
If the miners do hold the network hostage that would be a sign that a cartel has formed and the mining/fee market has broken down.

If that happens it may well be that the users and developers will augment the protocol such that the miners are once more 'held hostage' by market forces.

The difficulty evolution is often described as 'difficulty follows price', this does not model the cheapening of the hardware but otherwise describes the market forces pretty well (by assuming that block rewards and fees won't change rapidly).