I don't know anyone who has ever pitched an idea and the idea was not altered in any way by the time it was produced. If anyone says they have:

1. They weren't working for anyone else. So they didn't have to answer to anyone.
2. They are lying.
3. They suffer from blackout spells and memory lapses (get that checked out immediately).

Decision-makers may love your concept, but everyone needs to put their stamp on it.
1. You may not have covered the objective as well as you thought.
2. Even the best ideas have room for improvement. Don't let your ego make you blind to a great contribution or genuine constructive criticism.
3. Sometimes people need to justify their positions.

As you know, not all input is good. So if your idea starts to get dissected, you have to pick your battles. For some losses, you have to suck it up and move on. Fight for the core of the idea. If you fight for everything, you risk losing it all and dealing with hostile clients and/or supervisors in the future. (Good luck selling anything then.)  But when you manage to save the core idea, celebrate that victory. It's still your concept and it's coming together.