I feel like a lot of people in this thread are using the following logic:
1) Bitcoin has always been #1 because it was the first mover and it has the biggest network effect.
2) Therefore, "altcoins" are only interesting for short-term investments
3) Bitcoin will always be #1
I would just like to explain my view and why I think this circular reasoning is not very useful for predicting the future:
When Bitcoin was born in 2009 it was competing with fiat currencies. Relative to these fiat currencies it had so much going for it: decentralisation, 21 million coin cap, super fast, super secure, super cheap. It had tremendous benefits over every other currency.
In the last 8 years a lot has changed though.
For starters, the Bitcoin network has become very congested with fees currently skyrocketing and Bitcoin still not being able to scale.
Compared to fiat currency, Bitcoin is still a tremendously valuable asset. Even with fees of 100$, it could still be a very attractive store of value for people in certain countries.
However, it's no longer Bitcoin versus fiat. It's decentralised digital currencies versus fiat. And although Bitcoin is still #1 in marketcap, one has to acknowledge that a trend is occuring:
https://coinmarketcap.com/charts/ If you scroll down the the bottom of that page you can see that Bitcoin's market dominance has dropped below 50% now. Notice how the biggest drop in Bitcoin's market dominance happened right after the blocks got completely full (1MB), negatively affecting user experience, in March/April 2017.
You could argue that that's no problem for Bitcoin because the price per bitcoin itself still went up. But I think the price of all cryptocurrencies are going up because it's the future of money and people are realising this. Looking at how this new investment spreads out over the different coins is a much better metric to check the sentiment within the cryptosphere than looking at just one coin's price.
Now some of these coins really boggle my mind. For example, I don't get how XRP made it all the way to #2 on the marketcap list. I read their whitepaper and I really don't see how this is even closely representing what decentralised trustless currencies stand for. But hey, I might just not get and miss out on it's potential, and I'm OK with that.
But other coins really interest me because they offer some promising solutions to the problems that we ran in to while conducting our first decentralised currency experiment (Bitcoin).
Some solve the governance problem by using DAO's (Dash, Decred).
Some solve the scaling problem with faster blocktimes or flexible blocksizes (Monero, Litecoin).
Some try to make the currency more programmable (Ethereum, NEM).
Some solve the anonymity/fungibility problem (Zcash)
Ofcourse trade-offs are being made and I think none of these coins are as secure as Bitcoin is right now. Mainly because Bitcoin's software has been run and tested for 8 years now, but also because it's more conservative with it's goals (i.e. just being a store of value).
Long-term however, some of these "altcoins" (I don't like that name because it differentiates between Bitcoin and any other cryptocurrency) might take over. And the longer Bitcoin takes to resolve its issues the bigger the window of opportunity for another cryptocurrency to take over.
Let's also not forget that Bitcoin paved the way for other cryptocurrencies. It takes a lot of effort to get a store to accept Bitcoin if it has never heard of cryptocurrencies before. Once it already accepts Bitcoin, and a cheaper, faster and maybe even more secure cryptocurrency shows up, all it takes is a software update.
The way I see it is that the whole cryptocurrency community is betting on which currency will have what it takes to go big. Betting on just one horse might have been a good idea a while ago, but it certainly isn't today.