This begs the question, what if we had only Bitcoin? Wouldn't the level of adoption outgrow what it currently is at the moment?
I somehow doubt that Bitcoin will have this level of propagation if altcoin does not exist. Altcoin somehow helps in the adoption of Bitcoin because with the altcoin's marketing effort, they tend to carry Bitcoin in their marketing activities. Besides most altcoins are paired with Bitcoin in trading platform. This somehow gives extra exposure to Bitcoin. Aside from that, not all who are in cryptocurrency tends to know Bitcoin first, there are people who happen to know altcoin first before they get to know Bitcoin.
Do you think that altcoin is a major distraction to bitcoins adoption?
I do not think that altcoin is a major distraction to Bitcoin adoption, if there is a major distraction, it is the government that forbids or bans Bitcoin. Altcoin helps in Bitcoin propagation due to trading pairs and a all altcoins are paired with Bitcoin on exchanges. Aside from that most of the altcoin marketing activities often mention Bitcoin as an example.