I don't understand why the licenses seem to be 'forever'. Surely a 25 year license which is then sold again at auction would have been a better design.

The 'forever' licenses also can't be resold (although the entire company owning them can merge). That means if in the future another technology which completes with 3G so incumbents don't want to get involved comes along and could make great use of that spectrum, it won't get use.