Not so long ago there were standards for wide receivers in football( in fact, there were standards for every position, like a QB should be 6'5", or a DT should be at least 300 lbs). The tall, big bodied guys used to play on the two sides of the field, they used to be called X-s and Z-s, depending on which side of the field they were lined up.
And there were the shorter guys, who weren't necessarily the fastest players but had some decent agility. They had been called slot receivers.
Just think about any offense ran by Norv Turner. It was almost like they didn't even consider players under 6'3" as legit outside receivers.
But it wasn't just Norv Turner. It was how the game had developed through decades.
Nowadays there are no rules like this. And besides a few players (who are in most cases not the no1 or no2 receivers), there isn't any more slot receivers. There isn't X-s and Y-s either.
As an offensive coordinator, all you're trying to do (or at least that's how it should be) is creating favorable match-ups for your players. It could be a receiver lined up in the slot, it could be a bunch with several receivers, it could be a tight end or a running back, it could be motions, fakes, play actions, anything. All that matters is when the given play starts, your offense has a head start against the defense.
Normally the defense is always in a bit more difficult situation, since they can only react to what they see, what they saw before and make adjustments according to that. But in recent years, they job have become even more challenging.
Offenses often have at least 2-3 reliable players besides the QB, and trust me, the offensive coordinators (the good ones) will find the weakest parts of the defense, and they'll create matchups for their best players against the weakest members of the defense.
Let's go back to my slot theory: usually it's more difficult to make big plays in the middle of the field, since the traffic in that area of the field is obviously higher than close to the sidelines.
Usually if the defense is in a base formation, a safety or a linebacker is going to need to cover the slot. So if the defense is used to concentrate on eliminating the big plays, which more often happen on plays attacking the sidelines, it gives the defense a false safe feeling about the middle of the field. But 3 short passes for 10 yards each for the slot hurts just as much as one big play for 30 yards outside the hashes.
Offenses started to use the no1-no2 receivers in the slot, creating favorable match-ups with it. If the defense puts one of their starting corners in the slot, you'll have more chance to attack the sidelines. If they put a safety or linebacker on the slot, he's gonna get open. Of course, if the defense makes changes, like they're going to play in Nickel or Dime, you'l have a better chance running the ball.
So what can the defense do? As a matter of fact, they've been adjusting to these and every other changes as well. Is the name of Chris Harris Jr. familiar to you? It should be, he's been playing at an elite level for a long time. Oh, and he's a corner. A nickel corner (or as a lot of people say it for my frustration: a slot corner). So... Are you going to pay 3 corners starting salary? Maybe not, maybe you should just pay two and have a guy on a rookie deal for the 3rd spot. But just like there's no need for a specific player in the slot for the offense (because you could put there whoever you want to), there's a need for a good nickel CB in the defense.
In the offense, you're trying to create those favorable match-ups. But in the defense, an outside corner has different tasks than a nickel. As a nickel, you're going to be targeted a lot during running plays, and you'll need to be physical enough to be a good tackler. On top of that, from time to time you'll need to be able to cover top receivers, running backs coming out from the end zone, and even tight ends.
Defenses has been adapting to offenses ever since, and the more nickel and dime formations used are clear evidence that they've been adapting to offenses which are having 3 WRs on the field more often.
It's kinda funny that the answer to the extinction of the "simple slot WR" could be the birth of the "simple slot corner".
In the long term, I believe it's going to vary from team to team, but more like from system to system. A lot of teams are looking for CB/S hybrid players, even for LB/S hybrids. With that, they could have 3 safeties on the field at the same time, who are valuable against the pass and the run as well.
