Perhaps I'm missing something here, but surely if any airline's operational requirement/plans dictate that they need aircraft the size of the 42 then they number the fleet accordingly. They are not limited to the fleet size they currently have on strength.
I understood that EI-BYO was leased from NAC so surely it could be similarly replaced if it turns out to be a write-off, should a four-strong 42 fleet be deemed necessary. It strikes me that a mixed 42/72 fleet still looks eminently sensible for their route network.