The dominance is quite evident in the bull run led by bitcoin every time likewise when the price drops, all cryptocurrency drops including etheruem. What more are we saying if it is sure with the leadership of bitcoin. The price and volume trade is another factor showing the supremacy of bitcoin in the market.
Volume will always be high - BTC pairs are almost always necessary. Major bull runs always end up with BTC either staying stable or dropping a bit for some time and meanwhile, alts drop from 100% to shit in no time and stay there until BTC charts become boring.
We'll never realistically need so many coins and that is why I think the realistic percentage of dominance is way, way higher as most of the other "competitors" are just useless.. Hadn't they had insane pumps and dumps, nobody would even use them at all. Can't think of an instance where I contemplated using an alt for anything besides trading and multiplying my BTC.