I'd say this analysis is a little negative.
Since being electrified in the 90s, Kings Lynn has had the luxury of having its rolling stock replaced not once but twice.
1992-1997 - served by class 317s - oldest 15 years old at replacement, newest around 10 years old
1997-2017 - served by class 365s - new to the route, just over 20 years old at replacement
2017-2022 - served by class 387s - 3 years old upon arrival, now 7 years old
I realise GA have run 317s up there at times during that, however by far the mainstay of the service has been provided by WAGN/FCC/GN, and since electrification these services have never seen a train older than 20 years. Notwithstanding the ironing board seats in the 387s which we know at least one particular well-known user dislikes, I don't think anyone can say Kings Lynn has had a bad deal. Since 2020 Kings Lynn also has had the luxury of virtually solid 8-car service, which again is by no means a bad deal.
For the bulk of the above time the GN service to Kings Lynn was solid 365, which like the 387s were pretty high-spec when first introduced, though we know they were "dumbed down" somewhat during the 2010s refurbishment.