Under what assumptions is Ethereum a centralized shitcoin? There are at least several arguments you could make to argue that Ethereum is more decentralized than BTC.
It's because of the game theory associated to premined coins. I already touched the topic a bit in
this post.
If a coin has a large premine like ETH, this puts a "company" (or, in Howey Test vocabulary, a "common enterprise") in the center of the whole coin's economy. This "company" (which doesn't have to be registered as such!) is the team of developers, founders, early bird investors and ecosystem partners who benefit directly from the premine, either because they got allocated coins or because they received fiat (or other cryptocurrencies) selling premined coins to ICO investors.
There are particularly two centralization risks associated with that model.
First, as investors have suffered a dilution due to the premine, they expect the "company" to deliver work - development, marketing, and also a certain "spiritual leadership" (like in the case of Buterin himself) to make the coin flourish and increase its price. If the company ceases to deliver that, the coin will very likely suffer problems (it won't die instantly, but probably lose a lot of value). In such cases it is very difficult that the community "takes over" development of a premined coin. Why should you "work" for the coin's success if you haven't been benefitted by the premine?
This problem leads to a situation where the "company" always takes the main decisions regarding the protocol. They can involve the community but someone of the "company" will often have the last word. The community depends on the company's success.
Second, the "company" often holds lots of coins during extended times. These coins can be used to pressure for some (forking) protocol changes. In the case of Ethereum, the most egregious case was the ETH/ETC split. Buterin threatened to sell coins of the fork not supporting his changes, and thus he achieved consensus for the protocol changes which froze the TheDAO hacker's funds (a case of blockchain censorship, even if the hack was obviously malicious, but blockchains should be neutral).
There are probably more centralization risks but these are the main ones which prevent me to trust premined coins like Ethereum in contrast to decentralized, non-premined coins like Bitcoin. I always associate these coins with a "company" which is close to a single point of failure or "middleman" because they have more power than any other entity in the power game of the coin.
In Bitcoin, instead, we have an equilibrium between miners, hodlers and other "economic nodes", developers and "opinion leaders", which changes with time and prevents anybody concentrating too much power.