[Nursie]

>> Currently it's the same with BTC (higher price creates an incentive for more mining).

It's not quite the same, the act of bringing more miners on board does not feed back into the supply chain, it just increases competition for BTC that were going to be mined anyway. There's a missing link there.

>> The size of fees is regularly adjusted to make it worth their while.

The size of the fees is voluntary for the person sending the transaction. We have already seen situations where some pools started leaving some transactions out of their calculations because it wasn't worth their while processing them. As rewards for block discovery drop this will get more common and fees may have to become more generous, or somehow formalised.

[geoka9]

But some clients (electrum, for example) refuse to send the transaction if you don't include a fee? There's also this:

A transaction may be safely sent without fees if these conditions are met:

    It is smaller than 10,000 bytes.
    All outputs are 0.01 BTC or larger.
    Its priority is large enough (see the Technical Info section below) 

Otherwise, the reference implementation will round up the transaction size to the next thousand bytes and add a fee of 0.1 mBTC (0.0001 BTC) per thousand byte.

[Nursie]

Sure, those are some client implementations, not all, people are free to interact with the network in all sorts of ways. Servers are also free to not include transactions that don't have enough fee for them.

I'm sure when the time comes something will be worked out, but it is most definitely voluntary rather than an enforced feature. Hell, do a quick google search for "Bitcoin no fees" and you'll find thousands of BTC supporters shouting about how it's so awesome as a payment method because there are no fees!