My use case for my ipad is situations in which an iPhone screen is too small but a laptop isn't portable enough. That's not many situations, and a 5.5" iPhone would be "big enough" more often than a 4" iPhone. I think I'd keep my current ipad but not replace it in the future.
Of course, as others have said, it would depend on whether a 5.5" phone could work for me at all, and I doubt it could (due to not fitting in a pants pocket).