Would you call Seal Rock "alive"...?
Because that's the best case scenario if you just bumped them up. Note: Best case. Worst case, queue times for everything increases because the small community gets spread out more.
After all, Seal Rock already has all the rewards Shatter has, except the mount which is replaced by the Field Commander set (Which I personally managed to get about a month or so ago, so it does still pop - Ironically, though, it often stopped popping in the EU evening hours, which I suspect is due to the game waiting for a 72 match instead of going with the 24 people it had previously).
The same would go for Slaughter and Secure if you just bumped it up in levels. We already have a precedent of a "dead" frontline that doesn't have a different level/considerably worse rewards. So I can tell you that this is not going to solve anything.
