All our installers always had an online part at the end, sometimes the installer can't really estimate the time needed correctly, but the download should be usually fine.
In case the server is overloaded (likely, on release day), simply install with the network disconnected. The online part is only needed to be sure you are getting the current version of the Addon Manager but, of course, since the scenery is brand new, the online version and the one included in the installer are exactly the same so, you can safely install offline.