Not sure why we try to define successful by nothing more than the subscribers when neither company releases official numbers anymore so it is all speculation.
1. Does it have a healthy playerbase supporting a healthy economy
2. Does it generate enough revenue to keep growing and expanding
3. Do players still care enough to complain about it 24/7
That seems like a far better checklist for successful. Though I would say at least when it comes to healthy economy, that is more server based than anything else considering both wow and ff14 really could benefit from an overhaul with datacenter or region wide markets.