Even in conventional aircraft a parabolic flight plan (the vertical path) is optimally efficient, all other things being equal. In practice it'd be a nightmare

Even in conventional aircraft a parabolic flight plan (the vertical path) is optimally efficient, all other things being equal. In practice it'd be a nightmare to separate traffic. Additionally it'd increase the pilot workload significantly considering the cruise phase of flight is typically the "ahh, I'll order some coffee from back" time as the primary duties then are (a) monitoring aircraft state (i.e., scanning the gages) and, (b) changing the radio frequency each time the flight moves into a new sector and checking in with ATC. [me: aircraft dispatcher for a US carrier in a previous life]