If that's your reasoning, then Triple H vs. Undertaker 3 has been hyped all year as well. It wasn't long after Mania that Triple H himself said that when Taker returns, Triple H will be waiting. So if that's your reasoning, then you might want to reconsider HHH vs. Taker as the main event as well.
Not to mention how many times has WWE spent countless of money hyping yet another famous person coming in to "wrestle a match" only to put that match in the middle of the show? Floyd Mayweather vs. Big Show got WWE the most publicity and revenue in years, only for it to be in the middle of the show. Lashley w/ Donald Trump vs. Umaga w/ Vince McMahon was hyped for a long time as well, and it got WWE even more publicity and money. Was it the final match on the card? No, it wasn't.
If those matches weren't put at the end, after all the money was invested into those rivalries, what makes you think this will be any different? At this moment, Dwayne is just another actor going to the WWE to promote a match with WWE's next famous person. At the end of the day, Dwayne's no bigger than Donald Trump or Floyd Mayweather, and he damn sure isn't worth more money than Snoop Dogg who only usually gets a segment at best on a Mania card... or a clothesline on Santino, which ever's clever.
I'm sticking by the choice of HHH vs. Undertaker because in the end, this is the true match people are going to want to see. There will always be room for another Hollywood Actor vs. WWE Superstar on the show, but there will never be another chance to see two Hell In a Cell masters lock up for the first time on the biggest stage of them all to end one of the biggest rivalries of all time. That's everything you need for a WrestleMania Main Event.