Shounentai is more the exception than the rule when it comes to older Johnny's groups. There were plenty of the earlier groups that broke up, and more more semi-recent examples there's Otokogumi and Hikaru Genji that broke up after members left. So it's not like it hasn't happened before, just that it hasn't happened for a long time.
And IIRC the Hikaru Genji members that left did so to go solo so Jin isn't the first for that.
I don't think Uchi will ever be able to debut. He'll remain as an active Junior like Kazama and Hasegawa but I don't see him being allowed to debut again, not after this long.
NEWS though from the start has had the feel of being Yamapi and his back dancers. If he goes I don't think Johnny's will see much point on continuing with NEWS as promoting Yamapi was one of the main reasons the group exists.
Hikaru Genji had major issues though (from what I've heard, the members really hated each other) and Otokogumi was forced to disband due to the scandals they faced.
If anything I'm just surprised it took so long for Jin to leave KAT-TUN, I could see from day one, they were not going to last as a 6 member group. As for NewS, like seca said, the group was really made to showcase YamaP and if he leaves, there's not much point in continuing it. If he did leave, Ryo would leave as well I'm sure and a 4 member NewS would probably be more active since those 4 might actually care about being active as NewS members (versus YamaP and Ryo who tend to distance themselves). And to be honest, I doubt NewS will be doing any more dome concerts if YamaP and Ryo aren't there. (I think KAT-TUN cancelling their dome tour is pretty sketchy -- the earthquake was a good excuse)
In my opinion, Uchi shouldn't be allowed to debut again. He was allowed to debut twice and he screwed himself over. It was all his doing. He's lucky he's allowed to be so active now as it is (with concerts and such). Given how the JE debuts have really slowed down over the years, there -might- be a volleyball debut this year and with Kisumai debuting, it's going to be several years until we get another debut.