Should: They will be left with a lot of 'What Ifs' if they don't

If you follow pro wrestling outside of the actual wrestling shows/promotions, then you know that the New Day and the Elite are also friends outside of their respective promotions.
They often game together and frequently interact with each other through social media.
Breaking Vince Russo and Jim Cornette update HERE.
Since they are already close friends and have similar interests, it wouldn't be hard to envision the two groups having one hell of a feud.
It could sell a PPV or several. It could involve titles or no belts. Just the thought of the two factions going at it would be salivating in and of itself.
And that's just one feud.
If they decline signing with WWE, they'll be missing out on feuds with guys like the Usos, Finn Balor, AJ Styles, Seth Rollins, Dean Ambrose, John Cena, Roman Reigns, Adam Cole, Johnny Gargano, Aleister Black, Velveteen Dream, etc.
While they have faced guys like Gargano and Cole before, they haven't done so under the WWE umbrella.
And the WWE storytelling has proven to be far better than it was for guys like Samoa Joe, Styles and Bobby Roode in TNA.
Yeah, all guys were TNA World Champs at one point, but WWE has booked all three pretty well initially.
Roode and Joe were both NXT Champions and Styles has had two runs with the WWE Championship. Roode also had a short run with the US Title.
Yes, it can be hard to trust the booking from the outside looking in, but the Elite is the Elite, not Curtis Axel and Bo Dallas.
The potential matches and feuds they could have had in WWE will be left on the table if they never opt to sign with the WWE.
They might be fine with that, but a lot of fans will be disappointed.
The fans may not be the first thing all wrestlers think about when they make career decisions, but the successful ones wouldn't have gone as far as they have without the fans.
Also, think about the potential of the Shield vs. the Elite or the Wyatt Family vs. the Elite.