You don't have other options to get to 2400mm or 1600mm. These are very niche lenses that most will rend for that one special occasion they know they'll be used. This to me looks very much like they aren't expecting to shift a lot of them so it is special tooling. By using the 400 and 600 as base lenses it has likely drastically reduced the cost while also making sure of parts availability.My issue with these lenses is big price difference between base model + extender and these models. If that difference is purely about manufacturing, I guess Canon said something like "The new tooling costs us $1M USD, let's divide that over the 200 units we're going to produce."
But the weight, focal length and price point are all things that put these lenses in the "not for me" category, so what I think is very irrelevant