[benologist]

Replacing you while transitioning you into a new role means they basically end up with two new employees instead of one.

You should take their advice and easily get the job you want somewhere else.

[fooboy]

I don't follow. Your point is that my departure will yield a new developer?

I don't think my departure would be a good thing for them. I'm on a small team, and my leaving would greatly decrease their capacity.

I'm also being encouraged to take on more responsibility while I'm still around, so I'm not getting swept out the door or anything here,

[benologist]

They're filling two jobs if you move - your old job and you in your new job. It's more disruptive than just bringing one new person in to fill one new job.

Their capacity issues etc are not your problem. Your problem is how to get from where you are to where you want to be, if you're happy with more responsibility instead of a different role then stay.

[fooboy]

To be fair my direct manager is supporting me on this--he's giving me more quantitative work and the work time to knock it out. If I stay it'll be because of that.