ETH is really a very good long term investment and unless you feel you are investing in the right alt, then there is no point letting some of it go. It is an interesting project and it will keep doing well in the market. So, at least that is one assurance of seeing it testing different level of ATHs. This however does not mean that investing in other alts won't be a good one, but you have to understand your area of investment.

Up to last 15 months, if you go for a study then you can easily find eth remains as one of good investment opportunity closer to how bitcoin is doing. So, holding and buying some more eth may make any person to richer in short period of time itself.

If alt is not in your long term portfolio then selling them into bitcoins and holding bitcoins for longer duration also must be one of the greatest decisions in my opinion.