The Flyers (and tbf, hockey in general) just don't do that. Imagine a 4th liner with a disgusting wrister. Or McJesus speed. Or a pitch that's virtually unhittable. Those guys exist in other sports because they're encouraged to do what they can, and try to make the most of the things they excel at.
But 4th liners in hockey need a very specific (non) skillset. That's why you can't play prospects there of course.
You have it ass backwards. What team has 12 skilled forwards with above average speed?
In fact, few teams have six forwards who are legitimate top 6 players.
So how do you fill out your roster and kill the 22 minutes a night played by your bottom six?
You can:
1) field mediocre offense players and have them focus on offense and watch better players skate rings around them, lots of guys like Gagner float around the league and have one or two good seasons in ten.
2) field bottom six talents, but pick ones who can forecheck and play defense, so while they may not score much matched against better lines, they won't be sieves either, and can eat minutes while better talents rest between shifts. A good fourth line doesn't bleed goals and wears down opponents physically.
3) field limited players and shelter them, which is what many teams do with 3rd line veterans who have offensive skills but are otherwise limited, get them easy matchups (harder on the road), limit their minutes, and spot their shifts so they can focus on offense.
(2) and (3) are what smart teams do, depending on their mix of players.
Every team would love to have bottom six players who could skate like the wind, but they have to have brains enough not to be liabilities as well (see Rinaldo, NAK's bad penalties, etc.).
In reality, unless you're an incredibly deep franchise, those bottom six players are a mix of unproven prospects who are mistake prone, aging veterans who need to be spotted, and tough forechecker types who can control play and maintain puck possession even if they can't finish.
On bad teams, your bottom few players are AHL plus types, one reason you're a bad team.