I'm under the impression the Broncos were rushed into making a decision. The club wanted Bellamy, but they missed out and had their meeting leaked to the media. From that day on, Bennett's days with the club were numbered and the Broncos had to find the best candidate. They could have either stayed the course and gone with Walters or Demetriou or head in a new direction with Maguire or Seibold. In the end, they've decided that Seibold would be a more dramatic departure and they've rolled the dice on a first year coach. Seibold isn't the ideal replacement, but it seems like the Broncos board is willing to dream big rather than play it safe which I think is important.
As far as previous success, it's certainly a concern of mine. If you look at the list of premiership coaches Trent Robinson, Craig Bellamy, Shane Flanagan, Paul Green, Michael Maguire - they all did great work in the lower grades. Most of them achieved success but even in the case of Robbo he took a team that won a wooden spoon the year before (Walters' team) and got them into the finals both years. I don't think it's the end all to be end all though, Hook was caught out at NRL level and Rick Stone wasn't much chop either.