For now, you are right. But my theory is that a PoS coin will seriously threat Bitcoin's leadership in the cryptocurrency world in the coming years. Bitcoin's transaction costs are pretty high because of mining, only that for now the price boom has "hidden" this fact. Now probably we are seeing the actual bear market as a consequence of the freshly mined coins being sold. Nxt and other PoS coins don't have this problem and thus can offer smaller transaction costs.
The lightning network will significantly reduce Tx costs on the bitcoin network, allow for instant confirmations, remove blockchain bloat, and allow Bitcoin to scale to Visa levels of TPS.
Look into it:
https://bitcointalk.org/index.php?topic=970822.0http://www.coindesk.com/could-the-bitcoin-lightning-network-solve-blockchain-scalability/There are some really smart people working on it and it really isn't that difficult to implement (soft fork and some Tx malleability corrections that need to be done anyways)
But this network does not erradicate the costs of mining. Because of the high costs, miners will mine only for profit. For now, they can make profit with the coins that are generated, but with the "cost" of a continous "natural" bear market because coins are sold and drive the price down. So there is actually a high "hidden transaction cost" still not reflected in TX fees.
In some years, the block reward probably will not be enough for miners to be profitable, so miners will have to rise the minimum transaction fees. The only other scenario I can imagine is that miners massively will leave the boat and the system will become less secure because an attack is less costly. A price rally only postpones the problem to the future. Even the most steep rally some day will come to an end and then all these problems will appear with even more dramatic consequences.
NXT, PPC, SLM, NEM, BTS and all the other PoS, PoB, PoI coins do not have this problem. NXT can offer 1-nxt-transaction fee forever and probably even less when price goes up.