Altcoins are so small compared to Bitcoin it is hard to believe that they have any effect at all. Do you have any evidence to support your claims?
All the altcoins as a whole make up ~20% of the market cap of bitcoin (at least they did at the beginning of the year - I believe they have overall gone down in value since then so I am not sure where they stand now).
The reason that new altcoins are allowed to be brought to market is because the market allows them to do so. If the market were to stop accepting new shit coins then new shit/altcoins will no longer be brought to the market. To date I have not seen one altcoin that brings anything useful to the market that bitcoin cannot provide.
I would also argue that each new altcoin takes away value from the other altcoins instead of from bitcoin