Good thinking. My guess is that Ethereum is just the first of its kind. If it is flexible enough it may survive but it's staying power depends on an awful lot more parameters and unknowns than Bitcoin.
So if you speak about 'platforms', the first thing that I (still) think of is Bitcoin.
I think of Bitcoin as a digital currency.
There are no smart contracts and you cannot build directly on the Bitcoin network.
Platforms that come to mind are Ethereum, MaidSafe and Shift.
You can build dapps (distributed applications) on these blockchain networks; that to me is how you define a blockchain platform.