Even if you were able to buy other cryptos directly with fiat, that will not change the correlation that other alts have with bitcoin and that's primarily because of principles of traditional financial markets. Given that majority of traders of #cryptocurrencies also happen to be traders in traditional financial markets, the principles of returns based on relative risk apply to crypto markets as well.
Bitcoin is the the absolute cryptocurrency for everyome. Something that has existed for nearly 10 years and now has proven technology and use case. Other #cryptocurrencies still have to prove themselves. Bitcoin is the risk free asset of crypto world. The risk of every other cryptocurrency is relative to bitcoin. If bitcoin turns out to be absolute fraud tomorrow, you can say goodbye to the whole set of cryptocurrencies out there.
This correlation will never break as it hasn't broken even for the likes of etheruem, the only other successful cryptocurrency out there. And trading of #cryptocurrencies has little to do with actual use case. Traders primarily use technical analysis to trade these currencies and in the absence of fundamental driven investing, correlation between currencies is unlikely to break.
And that strategy, while valid, annoys me to no end. I try to pick my coins based on the long term, actual value proposition. I have an unfounded hunch that the utility of a coin does affect its value, it's just that so far that effect is masked by speculative trading (I try not to think about the DJIA when I consider this). I have hopes that eventually coins with a strong use case can be used widely enough that the inverse will happen, and it's one of the reasons I try to encourage strong content here.
True, the utility of a coin should absolutely effect its value. There should be a fundamental valuation assigned to a project. I think some projects like ethereum and monero should have broken this correlation by now. But given that most money in cryptos is speculative or well, money that is short term and drives daily volatility is speculative, so alts are just seen as a way to get beta exposure.
I think one reason ethereum may not have broken this correlation is that one is yet to see how useful ethereum can be in the long run. True, a lot of projects have been launched on it but most have little to no use case truly speaking. Ethereum will find its own valuation once the dust settles.