I see it like this: Alex has decided that he wants to leave after the season 10. So CBS has decided that it wants to continue the series and do season 11 without Alex. Peter Lenkov had to figure out how to do it without him. So he introduces this new character, Lincoln and Steve will leave Hawaii and Five-0. At the same time, believing that the network for a while (let's say in a few months when he rests a bit) would be able to convince Alex for a return. But later for no reason CBS changes his mind and ends the series, and the finale of the series becomes the finale of the series, because it's too late to change everything.