It is precisely the opposite.

Without altcoins, right now, everyone would need ASICs to mine bitcoin. That would drive people away.

Altcoins give people with average or better computers something to mine.

Altcoins get people into cryptocurrency and staying interested in cryptocurrency without a huge investment.

Altcoins provide something for people to speculate on and trade their bitcoins for.

Altcoin values SUPPORT bitcoin, not the other way around.

When bitcoin crashes, altcoin prices also crash. They are tied together directly.