Theme appeal might be cyclical, but Playmobil isn't; and frankly their business strategy has seemed erratic since they lost the old leadership. Plus they've never been on the ball with trends. If they make a theme for a trend, they release it five years too late. We're not talking L*go here.
I can't speak about recent changes in Geobra's management, as I haven't been following its evolution over the years ( though I have definitely gotten an unpleasant whiff of political correctness, of late ). However, I think Playmobil not following trends, for the most part, is a
good thing, and contributes to making them a reliable, if conservative, value.
Regarding trends, the thing is, from a kid's perspective,
the world is new — because he or she was essentially born "yesterday"; which means that kids are capable of developing an interest, sometimes even a fascination, for literally anything you put in front of them ( Okay, maybe not math!

Although... ) —
if given the opportunity!!For example, buy a 6 year old Victorian sets, and stories, and they'll make a world out of it. Back to the Western genre, there is nothing
intrinsic about that theme that would make it unappealing to a kid ( especially to a boy ), given sufficient exposure. Heck, I recently got my 11 year old other nephew into
Prince Valiant comics ( from the 1940s! ) — and he LOVES it! Prince Valiant is his new hero now.
The Bluecoats ( ACW ), and
Lucky Luke are up next ( among
many other things; not just western, of course )!
Trend appeal is more something that is imposed from the outside — by marketers — so that manufacturers can keep selling different iterations, with sometimes
very minor tweaks, of basically the same thing ( 'Got to amortize them molds! ☺). Ex:
"Kids, forget about the Spy Team Commander truck ( which you got for Christmas merely 2 years ago ), ask for the NEW Galaxy Police truck instead!" ...Yet, 'a fort is a fort is a fort' or in this case 'an [armored] truck is a truck is a truck' — especially when it is THE EXACT SAME TRUCK, but with a different paint job!

...Sorry, Geobra, I don't care what my nephew says, I am not buying him the GP truck,
in addition to the Spy Team one! ( ...But I might be amenable to getting him the cavalry, to go with the fort!

).
Classic historical themes, though, besides their permanence ( meaning, for ex: Western sets will be good forever, ditto for classic medieval sets, but 'Novelmore', or even 'Ghostbusters', are ephemeral trends: there for 2-3 years, and then gone, or dead ), have the additional merit of opening an educational window into history; and
that, can become a jumping off point for kids to want to read comics and books about the period, watch shows ( with Dad or Uncle! ), etc. ...Cultural enrichment.
Note: Compare Playmobil to Lego's overall strategy. The
Thundercats-inspired
'Legends of Chima' theme lasted, what, 4 years? I believe the next sword and planet theme after that,
'Nexo Knights', lasted even less: 3 years!!! Meanwhile, consumers ( parents and kids ) are placed on a treadmill, trying to keep up with the latest trend. For example, little Tommy was delighted to get the Chima castle for his 7th birthday, but 2or 3 years later, his little brother, now 7, wants the Nexo Knight castle instead — even though they fulfill essentially the same purpose, play-wise! So the Chima sets are sold off, or simply forgotten. There is another phrase for 'trendiness': 'planned obsolescence'! ☺ Compare that to giving Tommy a classic medieval castle, and his brother a different set ( maybe even another castle if parents have the money )
also from the classic medieval line: this way, the world gets bigger, and siblings can speak the same 'language' — and maybe learn something about history, too! That's value.