Growth
For NCL's lack of growth, I'd suggest the following can account for most of the problem: Reduction of the easyJet based fleet. That has meant that routes like CDG,AMS and CPH which should each have been generating >50k passengers annually are absent. Then no LGW - that's 100k plus, and BRS being strangled by a poor schedule.
TUI and TC switching flying from three rotations per day e.g PMI, REU + IBZ to two rotation programmes e.g HER + TFS as the LCC's took their Western Med routes.
It's the 'bread and butter' services that have cost us. Forget United, that's just a dot on the map -'no bigger than SOU in passenger terms. Transat to YYZ was a loss of prestige only.
Why have BRS, EDI and now GLA grown faster. Simple - get a Ryanair base.