Altcoins can be a good and a bad thing for BTC. At first, they brought some sort of validation to the world of Crypto as a whole and made more people trust in the fact that Crypto is going to be around for quite a while so they should avoid being left behind by the bandwagon. Now, on the other hand, the emergence of better functioning coins as products of hard forks or just coin creation (Bitcoin Cash, ETH etc, it puts more pressure on a coin like BTC which is slowly beginning to look like a relic in the greater picture of how much development has been made to better Blockchain tech.