I think altcoins have had both positive and negative impacts on the adoption of Bitcoin and the growth of the cryptocurrency market. This mean, increased awareness and interest, which has led to more people investing in and using cryptocurrencies.
In other hand, there are so many scam projects and pump-and-dump schemes that have caused skepticism and fear, which could potentially slow down adoption.
For my opinion, altcoins have helped the industry by pushing innovation and expanding the use cases for cryptocurrencies, but they've also brought challenges like scams and market manipulation.
I believe that the presence of alternative cryptocurrencies has been a net positive for the industry, as it has forced everyone to adapt and improve.
Except for a few altcoins which have be a legitimate use case, like it is the case of privacy coins, and as such we can say they had a positive impact on the market as they move it forward, the rest of the altcoins on the market have a negative effect not only on bitcoin but the market in general and the rate of adoption.
After all how many newbies have heard about this market, invest in a shitcoin, lose everything and then leave this market for good? No one really knows the answer to that, but it would not surprise me if the number was in the range of millions.