Agree that 2400mm f/25 is not useful on a modern high-resolution body. You should consider that the 150-600mm zooms essentially have a 1.4x TC built-in to get from 400mm to 600mm, so just putting a TC on it is essentially stacking TCs (not exactly, but close enough to practical reality).
But I disagree that AI upscale with Topaz Gigipixel (or equivalent), is a panacea. Aside from the technical problems it can create, there are ethical issues as well. Unless the viewer expects the image to be artistically interpreted, then using AI in a photo that could reasonably be expected to be in documentary form is not kosher. But I don't mean to send this thread off in another direction, so I won't say more on that here.
Also, I didn't mention that the R5 can see both the EF & RF TC when stacked, and calls them both out in the EXIF, but it doesn't multiply the TC factors to get the total extension factor and effective focal length right. So that could affect AF and IBIS.