Well altcoins made several people earn and keep making people earn insanes ammounts with it,soo thats why they keep and will always be.The main fact is all altcoins has the xxx/uᴉoɔʇᴉq .I believe if were only uᴉoɔʇᴉq we could see some manipulation at the uᴉoɔʇᴉq value ,and well less people thinking its a scam project,but well altcoins are supporting the uᴉoɔʇᴉq earnings.
I think altcoins just preventing uᴉoɔʇᴉq to evolve and grow maybe i am not right but that's what i am thinking.
But I dont agree with you that the evolution of bitcoin is prevented by altcoin, but bitcoin's enhancements are sometimes based on new alt coins. But the core evolution of alt coin is only due to bitcoin. Really alt coin is nothing without bitcoin.