I would call that very dangerous and very likely a premature optimization. Nobody cares you are a platform until they use your product, and if you spend all your time building a platform, there will be no product to use.
The path requires far more finese than "always create a platform". You need to balance over-engineering with the ability to re-evaluate previous decisions.
I agree Google has wandered far off this path, but imnot sure they are on any more well defined path than Yahoo. They just happen to be earlier.
This applies to startups, less so to more mature companies who can and will throw more resources behind a project. After the prototyping phase, once you're reasonably sure of the direction you're going to go in, it isn't a bad idea to keep extensibility in mind from the start. It's more like premature architecting, if anything, than premature optimization, which is much less offensive (and not even necessarily bad).