On the positive side, the emergence of alternative cryptocurrencies helped to increase overall public interest in the concept of cryptocurrency and blockchain technology. By offering alternative options to Bitcoin, altcoins helped to broaden the appeal of the cryptocurrency space and attract new users who may not have been interested in Bitcoin specifically.
I don't see any direct connection between creation of altcoins and public interest in cryptocurrency. I think if altcoins were never created, Bitcoin would still be as popular as it is now. Bitcoin is still by far the most popular cryptocurrency, if you ask random people on the street to name cryptocurrencies that they know, I'm sure most people will only name 1 coin - Bitcoin. I doubt there's any signifcant number of crypto enthusiasts who first learned about some altcoin and only then discovered Bitcoin.
and if you try mentioning other alts, they are clueless if they have something to do with crypto market also. most people at one point have heard about bitcoin, so its popularity is no doubt at the top of other currencies. sure, there are pros and cons of the existence of alts. but with their presence, you can easily identify the difference between btc and these alts.