It is very difficult to accurately predict the future growth of ETH and BTC. However, although difficult, we can at least assume that the future growth of ETH and BTC will be even better. This is based on the history and adoption rate of ETH and BTC and also both Ethereum and Bitcoin have very strong fundamentals which are difficult for any coin to shake.
Perhaps, we don't need to predict it perfectly as it was unpredictable in the first place. But as we check on its history, it keeps on growing. Dumps are somewhat to happen and market crises come but despite these things, Bitcoin and ETH still gain trust and growing investors which makes it a way to recover even if it was slow. This is the reason why the community never gives up and is still optimistic about the future of these two coins as we believe that they will grow more and the adoption continues.
I agree with you that even though they cannot be predicted accurately, bitcoin and Ethereum have gained great trust among investors so the future of the two coins will continue to develop in future, this is evidenced by the history of Bitcoin and ETH where both continue, develop and experience growth very rapidly from time to time.