HR Dono thinks that the Magician at least should remain an antagonist!
There is a very beautiful contrast between the Magician and the Massacre Soldier's philosophies as was depicted in this chapter:
Should they fight to the end, HR Dono thinks this would be a most exciting clash of ideologies!
HR Dono is worried that Oda will have the Magician switch sides and try to join the alliance. In particular, HR Dono believes that the Magician's card(s) that enables one to surpass their limits will probably have a role to play in this arc. Considering the Magician's current location, HR Dono finds it hard to imagine that power being used to power up the Beast Pirates (especially if the Magician is defeated by the Massacre Soldier).
Perhaps, the Massacre Soldier overcomes the Magician in the contest of ideologies and that motivates the Magician to switch sides? HR Dono thinks that could be satisfactory.