We had a lot of pnd the last 2 years, and nothing was able to stay in higher levels, thats true, but thats because of the very long consolidation of BTC. This was just a very bad time for Altcoins as well, but if BTC is ready to go further (which is probably right now) the gains of other coins will also be more persistent.
I maybe didn't write that, but I also wanted to point out, that I just don't want to stay only in BTC as other coins will maybe surpass. That's why I suggest to sell only a small percentage, if a coin is really going up, just to compensate the invest. It may go down, it may get worthless, but it also may surpass BTC somewhen. I am talking about really good coins like ETH, XRP, maybe STEEM