No. Altcoins are not backed by BTC. Altcoins can be exchanged to BTC or some money. That is not backing.

If something is backing altcoins then it is the belief that alcoins will be money one day. Same with BTC. Problems with altcoins is that nobody needs more currencies than 1 in the world without borders. The idiotic assumptions that people will have one currency for bigger payments and another for buying milk and bread are really incredible. They will have 2 or 20 wallets, where will be different amounts of money, but there will be just one global currency (maybe not bitcoin though). People do not want to count more currencies, there is no one advantage of it.