True, Bitcoin can never really fail completely - but it can become much less significant than what it is today - and that is a failure. So, yes, I am one of those believing that that Bitcoin is still an experiment and that it can fail. It most likely won't fail tomorrow, it most likely won't fail next month, not even next year, but ... will Bitcoin have any significant position in 15-20 years? I seriously doubt so.
For me personally, Bitcoin has already failed. Bitcoin started failed already late 2016, as significant Bitcoiners started saying things like "Bitcoin is not for coffee purchases", "Bitcoin is a store of value, not a payment framework", and particularly "Visa and Mastercard works just perfect for payments". I had become a bitcoin trader by then, but I more and more started feeling disfranchised from the product I was selling.
I paid my eldest son with bitcoins for going out with the garbage - now I'm paying my younger son with physical coins and cotton-paper bills issued by our national bank for going out with the garbage. Yes, Bitcoin has failed. Right now Bitcoin works very well, but we have had big capacity problems in 2017 and in parts of 2016 as well, with Bitcoin transactions being unreliable and costly. The problems haven't been solved much, the most significant reason why bitcoin works out excellent at the moment is that the transaction volume has been reduced. Once it picks up we'll have problems again.
Bitcoin has many flaws, and in the long run I believe it will be better to support some technology designed and implemented from scratch rather than keep building on the Bitcoin protocol.
The biggest of all the flaws is the 1 MB block size limit. Well, it isn't really a flaw, it was not meant to be a permanent feature of Bitcoin. At some point someone has managed to plant the idea that this limit is one of the defining features of Bitcoin, at the same level as the 21M coin limit. The truth is that the 1 MB block size limit, when left unchanged, is a capacity ceiling making it impossible for the masses to adapt Bitcoin and start using it as it's meant to be used. Bitcoin adoption has been great from the end of 2016 and until now - but this adoption is 100% due to speculators buying Bitcoin in the hope that the value will grow even more, and they don't keep the bitcoin in personal wallets, they keep their bitcoins on exchanges.
Now on the "2nd layer solutions" ... we've had "2nd layer solution" around for a long time already and it has become massively successful, much more successful than Bitcoin transactions. Forget about Lightning, forget about Sidechains - the "2nd layer solution" is trades and internal transfers on exchanges. Balance changes in a proprietary database, not much unlike what's happening in the banks. I have big hopes for the Lightning network, but it will take many years before it eventually will dominate, and even when it will dominate the need for more transaction space will not go away.
I strongly believe we'll get some kind of a "flippening" at some point, and from then Bitcoin will just lose significance. I seriously hope that the new crypto currency becoming the significant one will be a good coin, but I'm concerned bankers will manage to launch some kind of "digital currency" and, with the support of national banks, plebs and politicians will become the dominant coin.
Yes, the network effect is very important - but for one thing, in the grand picture, Bitcoin is still a quite insignificant thing. Some analogies:
The PGP standard from 1991. It's a big, big shame that most email communication as of today still goes in plain text and unsigned. Despite being a dinosaur lacking a lot compared to modern encryption schemes, PGP is still probably the best way to encrypt emails. Bitcoin has probably become bigger than PGP already, but still it's not big enough. People have given up on PGP because even for those having PGP it's easier to send unencrypted emails. Similarly, if the will come up with more-easy-to-use-and-bigger-network-effect alternatives to Bitcoin there will be a very low incentive for the regular plebs to use or support Bitcoin.
The IRC-network - once THE standard for internet chat - it fragmented quite the same as Bitcoin has fragmented over arguments on how to continue the protocol development. We had the Great Split in 1996, and commercialized proprietary silo builders quickly took over the scene.
Facebook - remember, they were not the first on the scene when it comes to "social networking" - there were other services having the network effect before Facebook wiped them out.
Coinmarketcap does some effort on tracking the "bitcoin dominance". Bitcoin was totally dominant until March 2017 - always staying somwhere between 75% and 95% - but then it started dropping. In June 2017 for a short while it dipped below 33%, and it seemed like Ethereum would take over the market cap. But, the network effect was still not there - Bitcoin bounced back, and by some point in early December 2017, it was around 60%. However, the problems with Bitcoin haven't been solved - not at all - and the Ethereum network effect has been growing in the meanwhile. Bitcoin dipped back to below 33% for a longer while in the beginning of 2018. Bitcoin is on the way back up again now, while there are no capacity problems - almost 45% now. My prediction is that it will grow past 50%, but never again above 60%, and in the next slump it will go below 25%.
You could be right about never going to 60%. When Bitcoin adds smart contracts it will take some percentage off of ETH.