I guess it depends on your definition of quick. The Shield have been together for 18 months. That's not bad at all. I think there are different situations with each tag team or group. A lot of people want Bray Wyatt to break away because he is so much superior to his family. Similar to Cesaro.
I think the other issue is that we see WWE break apart so many tag teams to give the individuals single pushes that do not deserve it, and in turn we want them to break apart other ones for the stars that we think do deserve it. What ends up happening is a tag team division that is wrecked and a midcard division filled with so much talent that nobody can succeed.
I would love to see 6-8 tag teams be just tag teams for 5-8 years. Everyone sees what Edge, Christian, Jeff Hardy, Bubba Ray, JBL, Bret Hart, Shawn Michaels did or have done as single stars, but they wer in tag teams for years before ever going single. It's definitely not a bad way to hone your craft or become a backstage veteran. And it doesn't mean you can't experience singles success later on.
To your point about the longevity though, most people who come into the world of pro wrestling wanna get to the highest point available. In the WWE, that is becoming the face of the company or becoming the WWEWHC, not stay in the tag division forever. HHH said it best; if you are not here to be the World Champion, you might as well go home.
This isn't the 90s. Times have changed, everything moves too quickly now. The pressure is bigger now. I will give you an example:
-With the whole Twitter thing and the interviews and stuff, the Superstars have many more things to do on their daily basis and the management seems to be way more control freaks than back in the day. As a result, the pressure is way bigger on the Superstars, who want to get that top spot. If you keep them or plan to keep them in the tag division for 3-4 years, they might probably get disinterested, especially with all the responsibilities that are given to them by the management. The money is there, but eventually, this whole thing will grow on them and might just walk out, like Cm Punk.
Edge and Christian didn't have that many things on their mind back then. They had all the time in the world to succeed as solo competitors without having to care a lot about interviews and if they did any, they could say whatever they wanted. Imagine let's say Ryback and Axel nowadays being presented with a 4-5 years run. I don't know the age of both, but I could say they won't be around by that time anyway, but let's say they will. I can safely say they will be burned out by then. They won't have any interest to compete. They will have the pressure by the management to say whatever it is to be said at interviews and so on. I think you get my point.
It's a miracle that the Shield has been kept together that long and I believe it is solely because the three guys are extremely talented and were given the ball and ran with it. I wouldn't mind seeing the break up, but they can achieve more things as a group. My suggestion? Keep them together, let them run for single's careers/targets while being friends and a group. After WM31, make them separate peacefully and all, and 3-4 years later, they can all re-unite as a team or for a WM main event as opponents.