Bitcoin cannot currently scale. The Bitcoin blocks are going to be full again very soon. This will lead to skyrocketing transaction fees, slow transactions, failed transactions, and people choosing to use other networks to transfer and store value, because there are other networks that are more scalable, faster, cheaper, and have more features. Most cryptocurrencies are technically superior to Bitcoin . Bitcoin has failed to innovate because of it's hyper-conservative community and development team. This will lead BTC going the way of companies like Myspace and AOL . Bitcoin will always exist, but it's going to lose it's first mover advantage and grow much slower than most other top cryptocurrencies that are more innovative.
ETH also suffers from some of the same problems as BTC , though even with this, it will still likely perform better than BTC to the extent that it is eventually bigger in market capitalization than BTC . BTC and ETH have the advantage of having the most liquid orderbooks, and highest daily volumes, and largest market capitalizations, and are the most well-known. However, all of these factors can very quickly and very easily change. The scaling problem has no quick and easy solution on the horizon, and ETH suffers from practically the same scalability problems as BTC . While ETH will most likely perform better than BTC because it's not nearly as close to capacity as BTC , and has more features, it will probably still perform worse than many other cryptocurrencies that are even faster, cheaper, and more scalable.
Eventually, the is going to move to the cryptos that are technically superior , once more people start to realize that BTC and ETH aren't scaling at the rate of adoption.
There are solutions are the horizon, like lightning network and sharding, however they are not ready to scale with the needs of the market at this very moment, and that is likely going to lead to people decentralizing around faster and cheaper blockchains over the next few months, while development of ETH and BTC lags behind adoption. Leading to several cryptocurrencies being larger than both BTC and ETH.
The reason why I am posting this on the ETHBTC chart is because there is a perfect (log scale), and several other indications that signal there being a bottom here, and BTC ratios usually bottom out around the same time as USD price. However, I do not trade this pair, and have no plans on trading it because I think there will be far more profitable cryptos to invest in than ETH and BTC . I think by next year neither ETH nor BTC will be the largest crypto by market capitalization, but they will still be top cryptocurrencies and be much more expensive than they are now, and, ETH will probably be bigger than BTC in market cap.
Here's a similar "perfect" harmonic pattern that appeared on ETHBTC in 2017, right before the huge bull-run.
However, I do not believe Bitcoin will be surpassed for several years, if ever. Bitcoin already has the network effect and a huge head start getting integrated with traditional financial infrastructure. And it has a much longer and more proven track record in terms of security and stability, and therefore people have the necessary confidence to store large sums of money in Bitcoin. It will take years for another cryptocurrency to earn such confidence.
The way I see it, after next bull market, Bitcoin will still be #1, then likely EOS #2. From #3 on, it's a bit of a toss-up. And again, I will say that if Bitcoin does not satisfactorily solve its scaling issues, the chances that it will get surpassed will start becoming much more realistic not in the next bull run, but in the one after that.