If letting Steve Austin go was a mistake, then letting Lesnar go was catastrophic. Austin was a huge star, but he was expendable to some degree, since he was done as an active wrestler and, to some, the Raw GM/Sheriff schtick had lost a lot steam. Lesnar was a main eventer in the prime of his career. WWE were relying on him to work on top for years and years to come. It's been well-documented just how burned out Lesnar was when he decided to quit the company just a couple of weeks before WrestleMania XX. He hated the travel, he was worn out and his heart wasn't in it anymore. Plus, after dropping the WWE Title to Eddie Guerrero and being placed in nothing feuds with the likes of Hardcore Holly, he was souring on the creative direction, too. So The Next Big Thing decided to pack his bags and try and make it as an NFL football player. And WWE didn't really try to stop him, readily offering him a release that included an unreasonable no-compete clause (no non-WWE wrestling or MMA for ten years?). According to Brock's autobio, Vince's only reaction was 'you can't do this to me' - he didn't tell Brock how valued he was or how much he meant to him and the company. He was just enraged that Brock would up sticks so close to WWE's biggest show of the year. Letting Brock go was a mistake. Maybe they had no choice, or at least felt like they didn't, but it sure didn't seem like they wanted him to stay that badly (despite the endless creative possibilities they still had yet to explore with him).