No need to bet, they told us all of that in the announcement. The old engine was built to go beyond FE, it's not scalable or portable to mobile. Doing the work needed to fix the problems in GvG is cost prohibitive when only 5% of players play it, and less than half can even access it.
Doing the work is NOT cost prohibitive when creating something entirely new, accessible to 100% of players, and fixes all of the problems of GvG by re-designing Guild Battles to avoid all the issues and limitations of GvG.
The shiny new object is not a deflection, it's the shiny new object. Play with the shiny new object or stay with the rusty old one. Seems they'll give GvG one last polish, remove what rust they can, and leave it for anyone who wants to continue to play with it. GvG will see no further development, no new maps, and GvG eventually becomes the forgotten toy no one plays with. It's kind of there now, only 5% play.
Regardless of what you believe about the 5% number, there is no escaping the fact the over 50% of users are mobile. Their proactive response to an antiquated system that can't be ported is to recreate a new system for everyone. There won't be Some functions of GvG coming to mobile, or new maps coming to GvG.
Battlegrounds is what coming to mobile, and to everyone, it will have maps that use goods and units from all ages and all ages play together on the same map at the same time.
They have thought outside the box, gone for the gold, and brought us something that will hopefully be exceptional. I think they can. I think they will..