I hope you find the "slightest bit of sense" in one or more of those reasons.
- Juggling adapters is a headache, and not suitable for all users, especially those who use $10,000 lenses. I keep a $200 adapter on each $1000-2000+ lens. I have thought about grinding off the switch to make it hard to accidentally activate.
- RF will be supplanting EF over time, so fewer and fewer will have to throw away the adapter.
- The detachable adapter manufacturing cost is small ($20-40?) relative to a $10,000 lens.
- It's hard to sell a new $10,000+ lens that is of the "wrong" format and requires an adapter. Adapters are for lenses you already have.
- Having a detachable adapter means one product SKU in inventory, not two, to cover RF and EF customers. That saves much more than the little tube costs.
- As noted elsewhere, the adapter can provide the benefit of being replaced with a more advanced (TC, switchable, filter) replacement element for RF or EF users. More benefits to all.
I seriously doubt that Canon will design its RF lenses to be basically EF lenses with an adapter added to it. That completely eliminates the engineering teams ability to optimize the design taking advantage of the new mount.
Everything they have done for RF has been brand new designs taking full advantage of the new mount, resulting in entirely new and innovative lenses like the 24-70 f/2, and the much smaller and lighter RF 70-200 f/2.8L. The RF mount is the future. They said they will continue to support EF, and DSLRs for some time, and they might put out an RF-EF adapter with a built-in switchable teleconverter, which is cool, but they have shown no indication to prioritize the EF mount compatibility in their designs of RF lenses.... quite the opposite.