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