55 is the practical limit without adding a ton of weight to a split keyboard to keep it from falling over easily.
70 is ideal, but then to do it with enough weight, would make it cumbersome to move around the desk, which is something you would do with split keyboards, because of the different use cases which may spring up.
For example, if you got a paper document, you can put it in the middle. or if you're gaming, and you want a little more room for the mouse. so movement is a necessary bonus to split keyboards..
There's also the case of Multi angle tenting.. x axis is good, but the Ergodox is superior, because you can adjust all axis..