Shard mafias were not at all the reason arena crystals moved to GAC. They made the move for two important metrics to increase: time in game (GAC takes between 30 and 90 minutes depending on how sweaty you want to get, while it took maybe 1-5 battles for a total of 3-15 minutes to climb once a day in arena), and wider account investment (instead of just having to invest in the newest GL and their team, you now have to invest in ALL the GLs, their teams, and other S and A tier teams, their Omis, and their DCs).
Ships don't have mods nor Omis nor DCs and I pray they never will, and they're already required in GAC, so it's unlikely they're going to dissolve crystals from Fleet arena as they did character arena. Keeping crystals there still drives spenders to R9 fleet commanders and get the new shiny 'GL' ship on day one for 21k crystals.