Many corporatations I know have their own SEO/PPC team managing in-house campaigns, which were being outsourced at times.

I feel that they are right. Once they have found that Internet Marketing is not a one time game, but an ongoing process, they probably cut huge costs.
Outsourcing cost to a reputed firm vs. CTC of a dedicated resource is much higher...

This is probably because of the value degradation by individuals in this industry. Freelancers are still making good money, singled out from their past employers, but I feel they are at risk.

What can be the scenario in the long run (say, 10 years hence)?

I think every big and medium sized organization should have their own in-house teams as internet marketing needs constant reviewing and application of new ideas and thoughts which would be much reliable if one can control them from their own base than instead of outsourcing it somewhere else. The output also differs a lot according to the marketing done by various sources and one which is operated by its own organization.

The trend is towards outsourcing for a simple cost effective factor. Who knows what will happen 10 years from now? Technology has changed the world so dramatically that next 10 years is like impossible to predict. Who would’ve foreseen today’s world 10 years ago?