I think that if there is no alt coins then the exchanges will have less profit.
Bitcoin will rise too much (much more that it is now) and early investors will be happy

In my opinion, only few alts have purpose in Bitcoin ecosystem. I dont want to mention any specific coin, just Bitcoin need some anon alternative when you dont want to anyone track your transactions. Other than that, maybe some faster coin, but all others are just shitcoins if you ask me.

positives:
if it wasnt for altcoins, then mining pool owners wouldnt be making as much profit from the merge mining that happens. thus they would attempt to milk the miners more, which would increase the tx fee war, where miner are trying to get more reward.

if it wasnt for altcoins, then some of the exchanges wouldnt have a business. seeing as some of them do not want to handle fiat due to the fiat regulations, they would be nothing to value bitcoin against without altcoin.

if it wasnt for altcoins, then people would not have as much freedom to play around making their own code, and be more forced into the bitcoin consensus of the github, where change takes forever and freedoms of creativity are limited to the security of bitcoin.

negatives
if it wasnt for altcoins, some premined scam coins would not exist, stealing real bitcoin value and leaving users holding a bag of s**t coins

I think if Bitcoin going to make it is to have some challenges from Altcoins

Altcoins sure brought a lot of people to Bitcoin. I think Bitcoin wouldn't be much different without altcoins, but it would definitely have less users/a slower adoption. Altcoins also brought many people aboard thinking that cryptocurrencies was a get rich fast scheme and many fell for that... Making some quite rich.

I'm not really into alts but I think they have a side benefit. All the people who created alts must have learned a lot about bitcoin in the process. They may help write the next generation of crypto code

Altcoins are good for testing and for doing some research. And if something is useful it can be implemented in Bitcoin. Therefore altcoins deserve their space.
But the main reason why altcoins popped out from the ground like mushrooms was the crazy bubble in late 2013 of Bitcoin. From that moment on many devs just thought they could make a quick buck.And to be honest it worked out very well for a few.
But none of these altcoins will have any bigger effect or could be a real threat to Bitcoin.
A few however may have a good chance to exist beside our giant. Coins with stealth technology like Monero, Dash could be highly demanded in the future in "certain niche markets".

During the 2013 bubble people saw altcoins getting pumped crazily and bought Bitcoins to buy the altcoins with. Those altcoin buyers were pumping so much money through Bitcoin that I think it helped sustain Bitcoin's high price for longer than it would have held up if there were no altcoins.

As long as the currency has volumes, then it's alive. This goes for every alt and fiat alike. What you're saying is that gold is now worthless since we have cash. But in my opinion it's volumes that create demand. If there is demand, that means it has value. I see some altcoins succeeding along with bitcoin, some maybe even taking over bitcoin. But in the next 10 years its mostly about bitcoin. However if bitcoin raises, most alts that have volumes will rise with bitcoin.