That’s a hard move by SOGo guys, I hope they know what they’re doing 
Regarding NethServer, building SOGo rpm packages starting from their source isn’t so easy AFAIK
Nethesis would like to support WebTop and not SOGo on the 7 version, SOGo is still on NethServer but modules have moved on Nethforge and we’re looking for new maintainers. Who wants to step up?
Honestly, I was one of the biggest SOGo fans in the past, I made the first integration and first neth packages by myself, but now I think that new ways were opened and we can work very close to the Webtop dev team (news and demo about new version soon!) in order to build a great groupware, together!