That's not CI. Example with 3 branches:

   Trunk
   Feature1
   Feature2

If you nightly merge Trunk into Feature1 and nightly merge Trunk into Feature2 and test all 3 branches, you still haven't tested the integration of Feature1 and Feature2. With enough developers working on enough features the odds that at least one pair of features won't work together approaches unity.

People have tried all sorts of methodology and design, &ct. to try and keep this from happening, and they haven't succeeded yet. On the other hand, if feature branches get merged into trunk as soon as they aren't completely broken, and with a way to disable them, then you will very quickly discover that the features don't play well together.

Because that would be continuous builds on unintegrated-branches.

The point of continuous integration is that everybody is reconciling their work with everybody else's all the time, thus helping you find nasty integration surprises early and while you can still do something about them.

have fun handling 100x build volume...