If I draw Franklin's face on a piece of paper that doesn't become a 100 dollar bill. It would be a useless piece of paper.
That's the same with altcoins, when a shitcoin copies bitcoin it doesn't affect bitcoin's supply or the fact that it is deflationary. It would simply be considered a weak copy.
Same thing is true if some day banks and other institutes start giving out "fake" bitcoin's through loans, etc. It won't affect bitcoin itself.