Exactly - it's not Sound Devices, Nagra, Aeta, Zaxcom, etc. who you would expect to have that kind of functionality.
That said, with the amount of things in the 70D that are software controlled, they probably could do assignable channel routing with a firmware update if they really wanted to. It would actually be a really great marketing / sales point for them to be able to say "assign any input to any output" like the "pro" names mentioned above do. They would be the only one in the price class doing that as far as I know.
On the one hand, I agree that this is a low priced unit so you can't expect a nagra. On the other, I don't understand the pushback against proposed software improvements on our recorder.
The fact is that the DR70D already has a very basic mixer. Since it has a mixer, then you should be able to assign channels. Especially, since a) it's already done to some degree in the current mixing capability, and b) that's one of the things that the most basic of mixers does.
Anyway, the DR70D already has channel assignable mixing selectability between pairs (1<>2 and 3<>4). There's also user selecable channel assignment, but the choices are pre-sets. With these basic controls already available, it seems like there should be should be no reason to either a) add additional pre-sets or b) add user assignability.
To imply this is only the domain of high priced recorders is letting Tascam off the hook with a software update. I think it's clear that mixing capability isn't the domain of high priced units, otherwise Tascam wouldn't have included a mixer in the DR70D.
If possible, I want channel selectibility in a firmware update if Tascam will give it to us, but as long as people keep saying that you can only get it in high priced decks, then yeah it'll never happen on the dR70D for sure!