I agree with your statement. Just consider bitcoin as the main invitation to the Crypto currencies.
Only thing is that 90% is utter garbage and the other 10% have not proven their mettle yet in terms of scaling.
We still have years left before any form of mass adoption.
I think 90% is very conservative.
Do you count the hackable smart contracts as garbage? If so I would say 99% of Alts are garbage, with most of the others being unnecessary or useless.