It's been a while since I've done the math, but a solid line of roboports moves fewer items than 4 blue belts (which takes up the same amount of space), IIRC. And if you've got fewer roboports than a solid line of roboports, it moves even less throughput.

Probably the only time it's hard to beat robots is when you have something like a megatrain unload station--it's extremely difficult to squeeze more than two blue belts per wagon (one belt per wagon per side) without robots, and there's no need to balance belts with bots.