The thing is, I'm often happy to get extra responsibility on its own; it makes the work feel more meaningful.

In this context, I view responsibility as more of a level of operating rather than just "more work". For instance, I'd be very happy to mentor a more junior employee it's not something I'd expect a promotion for.

That's obviously very different than if my on call hours were doubled. Doing twice as much work at my current level is different than doing my work at a higher level (which I almost always want to do).

I'd much rather get called junior while doing senior level work than get called senior while doing junior level work.