As can be seen by the Bitcoin Dominance figure, this seems to be happening to some extent.
Sometimes we can see this kind of dynamic in the total coinmarket cap, and then we might get a kind of false sense that "it's happening" this time for sure, and then suddenly the alts and ICOs get pumped again.
Maybe this time it is for real? Even though ICOs and alts have been getting quite a bit of negative press recently, there still seem to be a lot of gullible peeps out there who are willing to buy into "supposedly cheaper" coins.
I think that I need a quite a long death spiral before I believe that the alts are not going to just parasite their way back onto any bitcoin recovery that might happen in the near future. in other words, it really seems that we might need a kind of long term blood in the streets before the parasites will release their grip on our bitcoin host... and I am not referring to bcash here as even close to serving as an arguable "host", because I consider them to be included in the parasite camp.
I think the more prolonged the bear market, the more potential for bitcoin to gain vs. the alts. BTC is better positioned to win a brutal war of attrition in which new resources coming in are limited.
Without new kids bringing their allowance and lunch money into the market every day, hoping fpr a "10 bagger", Dentacoin and Electroneum will fade away. Ethereum and Bcash may fare better, but their market shares will slip significantly vs. the master.