I personally hope that Brock vs. Rock isn't for the WWE Championship. Let's be totally honest, the title doesn't mean much to either man at this point in time and that's almost certainly not going to change. In my eyes, Brock vs. Rock in just a special attraction match, just as Cena vs. Rock at WM 28, is all you need. Just book the two guys strongly and put a good amount of focus & attention on their feud and things will work out just fine, probably a helluva lot more than that.
Besides that, WWE is waiting to see just how hot Lesnar stays. Unlike The Rock, we're not going to go months at a time in which we see Rock pop up on Raw. That means, Lesnar's heat might start to fizzle out before too long, maybe by the end of the summer, and plans change. While Lesnar is expected, at this time, to be involved in a special attraction match, it's FAR too early to tell whether the WWE Championship will be involved.
In some ways, I do understand the concerns of the OP. However, ultimately, it'll depend upon how they book Lesnar. Let's say, for instance, that Lesnar is booked to just have this super dominant run in which he tears through everyone put in front of him. Along the way, he happens to become WWE Champion and continues to tear through everyone put in front of him. The scenario that I'm most worried about, if this comes ot pass, is that the storyline will be The Rock returning to WWE to slay this beast that nobody has been able to stop. Then at WM, The Rock puts the monster down, becomes WWE Champion and "saves" the WWE from Lesnar's rampage. Lesnar's deal is set to end after WM 29 so Lesnar could potentially leave with a shitload of cash and another dominant WWE run, only for a movie star & former wrestler that, by that time, will have only wrestled 2 matches in the past 8 years to come along and save WWE where the current crop of guys failed. Yeah, simply put, that will make every other wrestler on the WWE roster look downright inferior. It'll make a lot of guys just downright look bad I think. I just pray to God that's not the direction WWE goes.
As far as the concept of the match between the two in and of itself, I've got no problem with it. Like others have said, it's just business. Such a match could potentially help to generate another massive WM ppv buy and lead to even more money for everyone involved, that includes the talents that probably won't be all that excited about it. Some feelings might be hurt and toes stepped on, and I don't bemoan any of the wrestlers for feeling that way. It's 100% natural to feel a little irked by the situation. Daniel Bryan & Sheamus have said that they're not wild about the decision for their WM match and how it came about. At the end of the day, however, they're professionals that did what they were told and came away from WM with a grand payday.