Wye, Why? Surely we want to promote no interlining in new stuff?

Interlining is not some objective evil thing. It's a tool for serving different areas with less infrastructure.

True. What about the mixing of circumferential and radial service? I should read your post again, but I recall thinking "what about building the bridge and extending the N to LGA instead?" and that wasn't one of the alternatives instead

I don't see how this is mixing service. This is two orbital services that split. Extending the N comes with a lot of problems and costs that don't really pencil out. I'm not saying this is a cheap cake walk, just that it works as a single project that solves multiple access issues.

to/from LGA feels radial to me, both in the literal sense of the trajectory points to/from midtown, and in the more generalized abstract sense of the different ends have different sorts of demand (clear destination vs source valance).

But, if there are specific problems with extending the N in your view, I didn't know that. That makes me understand this better --- it's less "I love Wyes" and more "we need *something* to connect rail to LGA to, the IBX works better than other options"

Extending the N requires both elevated and tunnel construction through a residential neighborhood. The curves to snake it through are also not great. IBX alt instead keeps construction along the highway, so less impactful, and light rail can take the turns better.