All altcoins can become a scam at any time, even those with strong infrastructure, utility, a powerful community, a large fund and a real product. Even ETH can turn into a scam under certain circumstances.
ETH is turning centralized, so yes, there's a high probability it will become a scam in the future. I'd say there's no altcoin you can trust these days due to their extremely-unpredictable nature. If there's only one coin I'd trust, that would be Bitcoin. It's the original cryptocurrency that started it all with a proven track record of development and innovation. With decentralization at its focus, I believe BTC will last for generations.
I don't see ETH becoming a scam due to increased centralization, rather it will become more controlled and subject to regulators. This would result in the end user having their coins censored and blocked at any time for political or other reasons. This is definitely not good for cryptocurrencies in terms of their fundamental properties. For speculators and traders it will not turn into anything, they don't care what they trade with, no matter if it is centralized or decentralized coin. The main thing is to have volatility.