This is a key question to try to determine the future of altcoins. I think altcoins will always exist, but if sidechains are a success, most of the serious projects will eventually run as sidechains, and the Bitcoin blockchain will be the bones of the whole thing, very robust bones.
I believe that is a singular approach towards problem solving. If Satoshi tried buidling a financial system on top of the existing banking system, Bitcoin itself would not exist. Radical thinking is what lead to the birth of Bitcoin. As a matter of fact when it got released people did think it was a joke, but look at where it is now. The point is, sometimes, building things on top of an existing framework could often times be limiting in a number of ways. Due to the same innovators tend to lean towards a chain of their own. Often times it is to serve their own personal, private interests but in a few genuine cases it might just pave a whole new paradigm shift.
Keep in mind, I am not defending alt-coins here, all I am stating is that side-chain are not the only way to go about innovating in the ecosystem.
In the book "The internet of money" - the author extensively writes about how the blockchain paves way for cryptographic currency to spread and eventually grow into a monetary system. You should really read it. I could reproduce an excerpt here but I am not sure that'd be legal.