If you are investing to have a return in after some years, then you should invest.
Establish how much money you would like to invest, and buy in chunks. One small percentage today, another tomorrow if price is better...

Some altcoins may start to be adopted... exchanges are also adding more and more fiat/altcoin trading pairs, which reduces the correlation between altcoins and bitcoin price, so you may diversify.
Moreover, bitcoin dominance is decreasing. It was more than 80% til march 2017, now it is around 50% (and it is decreasing again).