Diesel's main failure as a champion stems from two things.
1. Wrestling as a whole was severely down in 1994 and 1995 in the US. It's not like WCW was killing it yet either. Attendance was ghastly.
2. He was a giant 7 foot tall with five moves...and was a baby face. That means instead of facing Bret/HBK/Taker/Razor in matches where he could show how ruthless he was, he had to sell to credible looking heels. This meant the likes of Bam Bam, Mabel, Sid. Oh sure, HBK was heel most of the time...but you can't have him believably sell face in peril against a guy he towers over. So he had to face shitty guys and sell for them month after month. His two competitive matches as a face are a Face/Face with Bret at Royal Rumble 1995 which is a great match with a ton of interference and his match against HBK at WM 11 where HBK steals the show, making Diesel look kind of like a fool while also putting him over and cements himself as the next big babyface for next years program.
The second Diesel loses the belt to Bret and becomes a tweener he immediately starts having the best matches of his career against Bret/HBK/Taker.
As far as Reigns goes he probably won't have that sort of problem even if they went with him as a face champion. He's not that big so that he'd have to pull off a credible face in peril to the likes of Big Show and Kane week after week and he has a bunch of natural opponents already lined up. His problem is that they stopped booking him like an unbeatable monster a few weeks back and that the Rumble for many reasons didn't help him. None of his eliminations really mattered or looked good. They tried with Show and Kane but it just didn't make him look strong. 2004 John Cena would have put both of them on his back and tossed them out.
Business was in the shitter, but with Diesel on top, things were were than they were with Bret on top. Nash didn't have moves then - he had the same pat hand as ever, but he did a plancha every now and then. Nash was the lowest-drawing champion in many years at that point, and it wasn't just a matter of "well, business was down" because business COULD be hot.
LA was on fire thanks to the lucha boom, which gave WCW a big leg up for the next few years (that they largely squandered) while WCW itself managed to get a couple of good buyrates out of Hogan despite their core fanbase absolutely hating him because he was Mr. WWF and WCW and the WWF had radically different die-hard fanbases due to presenting completely different products.
Brock's only going to re-sign if he A - gets a gigantic offer that trumps his current deal or B - truly has no intention of fighting again. Brock's been a competitor for much of his life and it would be foolish for him to re-sign before at least knowing what UFC and Bellator, with Viacom's backing, are willing to pay him. UFC can give him greater fame and PPV points, but he'll lose out on sponsors. He can get those in Bellator, but have to trade in being for the #2 brand, but also having a chance at a title given how their heavyweight division is even more limited than UFC's.