To be honest - holding top spots in arena isn't an indication of server inactivity. We had 3 merged already (s11-18) and came from the newest server each time (18). We established our ranks in arena after every merge. We now hold most spots as well but have 2 very active guilds and 2 mediocre ones. Not counted the small ones.
When considering server merges they (Lilith) look at the whole server's active playing players.