That's a good point, but still I think it's inexact, because it would be very impractical to simultaneously use different web protocol, while it's really easy to set up more than one wallet and/or trade between different currencies. Cryptocurrencies are not really in competion like the different protocols used to be, which means that fluctuation in the crypto-marketshare of Bitcoin and other alts can vary greatly in the future and there's no reason one will stay high on top.
Also, the infrastructures built around Bitcoin are easily cross-compatible with other currencies.
IPv4 to IPv6 is mostly a software issue where computers can be upgraded and some switches/routers have their firmware upgraded. Why is it so difficult to switch when IPv6 is superior and in many cases a very needed upgrade?
The reason why is because while it is easier to upgrade user wallets(coin) or OS(IPv6) there are these additional problems :
1) IPv4 is still in many computers where users don't upgrade right away , ISP's and sites need to have a very long transitionary phase where they support both protocols.
Bitcoin has the greatest user adoption and in the most cell phones, computers, and most people are familiar with Bitcoin's companies /hot wallets. There would also need to be a very long transition because merchants aren't going to alienate clients.
2) IPv4 has tons of services and codebase dependent upon this protocol. IPv6 requires adding or modifying a lot of code not just in one company but across the whole economy. This is very expensive.
Bitcoin has thousands of developers writing apps, sites, and API's that interact with the bitcoin blockchain. Changing this is very expensive.
3) IPv4 is sometimes hardcoded in certain very expensive switches at datacenters and/or firmware coded switches that no longer have support from the companies to upgrade to IPv6.
Bitcoin has many companies coming out with Point of sale and hardware wallets that either cannot be adapted to another alt or won't.
A individual company can come and go but Bitcoin isn't a website or company but a whole ecosystem of hundreds of thousands of companies that are integrated with it.
Also, the infrastructures built around Bitcoin are easily cross-compatible with other currencies.
ASIC miners can only mine Bitcoin or very similar SHA256 alts. Most Cyrpto-currencies cannot be mined now or ever with the existing Billions of dollars in hardware.
Finally, Bitcoin is Open source and evolving and can swallow many alt features in the future directly or indirectly through sidechains/tree chains. Alts in general that aren't tied to Bitcoin like namecoin are in serious jeopardy in the near future because of this.
I believe there will always be hundreds of alts but besides a few of the 3-4 top ones most will radically shrink in proportional value to bitcoin as Bitcoin takes over and becomes more robust.