If I were a customer, and you'd reduce the feature list because the timescale is fixed, I won't pay you.

The timescale is not fixed if you can discuss the feature list, then the timescale. A fixed timescale means that your completion date is fixed before starting talks about what features you want.

You can't expect someone to implement an arbitrary number of features when the timescale can not move (well, I suppose you can expect anything you want, but you will also be sorely disappointed with reality).