My take on it has been that its not that young people have less or no interest, Its that young people are not attracted to the dry, humorless form of simulation that the elders have drifted to after having their own fun and games, years earlier.
No mission or reward structure is part of it, but its also a general community miasma/attitude where P3D users boast that they are not using the sim for anything as childish as entertainment, and are instead conducting "serious flight training following all the real life parameters that…
YAWN!!!
Opppps, sorry, where was I?
Its not that there is anything wrong with approaching simulation from that standpoint, but with it has come a tendency to look down/deride anyone not using flightsims in a very particular narrow way, and the attitude is palpable on many forums with causal references to not wanting things “dumbed down” for the “Console kiddies” who should "go back to their Xbox’s. Etc.
Those “console kiddies” are lurking and watching (like I did) and seeing that is NOT encouraging.
There is also how that attitude gradually removed the traditional “ramps up” and extensive training missions that were once standard for the genre, instead tending to assume everyone is at near PMDG level.
P3D completely got rid of missions for a while, and many in the community brag proudly how they never used an FSX mission. (those are for children!)
Dumping bags of flour? Ufos? Flying through rings? OMG!
X plane, with its original impenetrable engineers only user interface used to drop new users straight into the cockpit of a 747 with not an ounce of explanation, and if you are an MSFS2020 Alpha or Beta tester you will find numerous threads in the user forum from people howling that any hint of user friendliness makes this new offering a horrible kiddie-centric toy of a game…
Bleah! Why would youngsters pop into that atmosphere? Does it sound like fun?
So instead, its War Thunder that has the millions of players. Shrugs