Having altcoin means few of the marketcap that bitcoin could have going towards altcoins marketcap. So if there were no any altcoins than bitcoin marketcap could have been bit higher and also same for price.
Quite the opposite, altcoin provide an incentive for people to convert their dollars to Bitcoin because most altcoins are only tradable against Bitcoin. Altcoins bring a lot of new people to crypto and this is beneficial for Bitcoin. I don't think Bitcoin price would be anywhere near what it is today if there were no other cryptocurrency.
That is right. The altcoin attract more people as there are so many of them. When they know the altcoin, they will love bitcoin.