I think it would be better for the medium if the Spidey reboot fails, though. It would prove that rebooting is NOT a get-out-of-jail-free card, and that it can do more harm than good if the option is not used with discretion. There are TOO MANY REBOOTS right now. Franchises that actually NEED reboots, like Superman are being ignored, while lots of franchises that DON'T need reboots are getting them anyway just because studios think it's an easy and fashionable way to make money. If Spider-Man Begins bombs, studios are going to have to reconsider the reboot business model, and hopefully give more consideration to actually PRODUCING NEW MATERIAL and BEING CAREFUL ON THE FIRST TRY.
Spider-Man 3 made almost a billion dollars, and if Spider-Man Begins makes around 200 million or less, it's going to be a big slap in Sony's face. It's hard to imagine a Spider-Man movie underperforming that badly, but if people react negatively to the franchise being rebooted so soon after the Raimi movies, it could happen.
Rebooting is essentially dropping an A-bomb on a franchise and then rebuilding from the ground up. Sometimes you build something better, sometimes you build something worse. People in general loved Raimi's Spider-Man trilogy, to to see the house that Sam built being s*at upon already by the studio. The audience still thinks of Tobey Maguire as Spider-Man. He last played the role only 2 years ago, so his version of Spider-Man is still tied to the public's perception of Spider-Man. To suddenly see some Disney-Channel castoff in the role is going to be jarring for everyone including the oh-so-important kiddie demographic, and I think it could potentially hurt the film's box office potential.