When not in use, why not just keep it attached to the headphones?

> Citation needed.

Uhh, personal experience? I've certainly bent my share of 3.5mm plugs, but I've yet to hear of anyone actually bending their lightning cable plug.

> […] physics says the same amount of force would be better spread out via 3.5 connections […]

This is a specious argument. Physics does not say this at all, because you haven't defined the materials you're working with. If the lightning connector and the 3.5mm plug are made out of the same material, and the 3.5mm plug is completely solid, then you could make this argument. But in my experience 3.5mm plugs are not made out of material that's as durable as the material lightning cable plugs are made from, and 3.5mm plugs also often give the impression of being hollow inside. Sure, you can probably make an extremely durable plug, and I'd guess that more expensive headphones probably have more durable plugs than cheaper headphones, but regardless I don't think you really have to worry about bending your lightning connectors.