Another product of a discussion with my father.
We played Dark Heresy (1e) together for a short campaign or two. We more or less agree that it's a fun premise for a game, but a lousy system.
Meanwhile, Traveller is a pretty decent system, but it's hobbled by the fact that its implicit setting and expected conventions of play are unfamiliar. There is no Appendix N of fiction in the Classic Traveller books, unfortunately. So people misuse it for Firefly when it was meant for Space Viking and Dumarest (or so I'm told). About the closest I've read, works of that era that get at the "Imperial Science Fiction" feel, might be Dune or Foundation.
But it's not quite true that Imperial Science Fiction has no heirs. Warhammer 40k is also Imperial Science Fiction of a sort, though a deeply pessimistic lens. You have a sprawling, strongly human-centric empire with subsector and planetary governments who are only loosely under the thumb of central power - just like Traveller. You have faster than light travel which is slow, unreliable, and leaves ships isolated from each other for the duration, just like Traveller. FTL communication exists, and it's faster than FTL travel, but still not great in either. All three of these are features which permit characters out on the fringes much more autonomy than would otherwise be expected. Psionics are rare, dangerous, and stigmatized, much more so than in Traveller. How many science fiction settings have psionics at all, nevermind agreeing on the general attitude towards them? Ship ownership by private individuals, the Rogue Traders, is rare and inherited, versus ship ownership being rare and just extraordinarily expensive in Traveller (I guess you could do multi-generation-term inherited starship loans...). Powered armor (and the skills to use it) is also very rare and expensive in both, while both also have very heterogeneous mixes of tracked, wheeled, and grav vehicles and both energy weapons and slugthrowers in common use. The emphasis on melee combat is surprisingly high for science fiction in both - "cutlass" is a legitimate weapon choice in both. And the position of anti-aging treatments in both settings has some similarities; they exist, but they're rare, expensive, for the rich and powerful, probably not for your character.
I recall hearing tell that Warhammer 40k's setting had its origins in Traveller's universe, but filtered through a British black comedy 2300 AD / Judge Dredd lens. And the more I think about it the more right it seems.
So the natural conclusion is that Traveller is probably a really darn good system for running Warhammer 40k RPGs; almost certainly better than the baroque percentile monstrosities that have been churned out in the last ten years. And if you want modern gamers to understand Traveller's default assumptions, you could do a lot worse than describing the setting as a lot like Warhammer 40k, but less grimdark, with all the craziness dialed down to like a 3. Yes there are space marines in powered armor and they'll probably ruin your day if you try to fight them, but under the armor they're just well-trained dudes, not centuries-old super-soldiers. Yes the planet is a feudal technocracy and it's sort of like it's run by the Mechanicus but they're not as culty and not as cyborg.
The real question is whether you'd use Striker or the actual WH40k wargame rules to resolve small mass combats.
And hey, nothing says "To be a man in such times is to be one amongst untold billions. It is to live in the cruelest and most bloody regime imaginable" quite like death in chargen.