Define plenty of players? At any given time the amount of players in the NHL taken after the 3rd round is around 15%. The probability that you get a player that plays more than 100 games at that spot is in the single digits.
There's two end games to this logic: either these picks are low % and, by dint of reductio ad absurdum, you should
always trade them......or you pick in volume, increasing the %s.
You're right in that 4th rounder probably ends up nothing. But you don't know that -- no one knows that. Every team in the league, especially the good ones, is littered with players selected from the 3rd-7th rounds (or undrafted!). Not just depth players. It's part of a larger theme in that this team has no initiative to acquire high 1sts, while also not selecting in volume. That's a dead end. Carolina
just turned two late 2nd prospects and a 4th round prospect into Jake Guentzel. And these picks are liquid; they can be used to trade up, trade for a more positive depth player, whatever.
What the Flyers have done in the 4th round specifically has as much future bearing as what they've done at 2nd overall. Or looking at the last dozen 16th overall picks to prove no one good gets selected there. Or saying the Ristolainen trade was fine because of who the Sabres took. The good players
are there, and you can do whatever the hell you want with that pick.
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Separately, no one disagrees that veterans are important. But they do need to clear a bar of being NHL caliber. Everything you're arguing, doesn't Marc Staal qualify? They got him for free. And he's bad. But better than Johnson. Honestly, what exactly is Erik Johnson imparting in a month+, while the plan is to also (hopefully) sit him when they're healthier? Trading 4ths for burnt to a crisp veterans isn't a winning strategy.
Matt Niskanen *drink shot* was everything you're describing: recent Cup winner and mentor, while actually being the best defenseman on the Flyers. Result afterwards? 3 years of regression from all their young players. Bad teams overstate things like this too.