I have been looking at *coins for a while and the implications of the blockchain-based concensus system are interesting. The system really maps the real world onto computers very well, in that it reduces what used to be technical issues to "political" issues. These systems work on concensus, and as such, require a significant amount of interested parties to work in the expected way. They are very much subject to netw…
Building on top of Bitcoin is the easiest and simplest way of accomplishing this, because otherwise every new app has to build and maintain its own blockchain in a low level language like C++. In which case, core Bitcoin development gets increasingly fragmented and everyone and their mother has an altcoin that then needs to be marketed on its own right to miners and to the public.
While it's true that the Bitcoin blockchain would grow in size if more apps built on top of it, the Bitcoin blockchain will grow in size regardless of what anyone thinks or does. It's already 15GB, which is a far bigger burden on consumers than what they're used to.
There's always a huge backlash amongst Bitcoin developers whenever someone tries building anything on top of Bitcoin. Timestamping or notarization? You'll get yelled at for the mere suggestion of including this "trash" in the blockchain. Very, very unfortunate IMO and a huge mistake by the Bitcoin development community.
I honestly think it's because they'd rather stick their head in the sand about the blockchain. The fact that the Bitcoin blockchain is growing faster and faster disturbs them because it means not everyone can run a full node on their Macbook. I don't think it's any reason for concern.