Even if your "test-net" theory is right, there is always going to be needed to test some new feature... I hardly doubt segwit is the ultimate BTC change... Also altcoins are useful for many other reasons than testing features...
What happened today is due to people buying BTC before august 1st, in order to have 2 coins... After that, altcoins are going back up, and BTC will probably fall because everyone is going to be buying cheap altcoins with their bitcoins doubled...