To some extent, Bitcoin's price is effected by altcoins'. If altcoins disappear, Bitcoin's price would be higher. More money will stay at bitcoin! No shitcoins, no dump and pump, it will be good for the image of bitcoin!
You missed the other side: how much fiat has been brought in to the crypto-economy (such as it is) by people who bought Bitcoin solely to buy their favourite altcoin(s)? Overall, it might well be a wash.