Why would they? They have always followed the market leader and have recently gone multiplat. Accepting a moneyhat from Sony would be detrimental to the growth of the IP and the sales of DQ XII and future installments. Square Enix executives like taking checks for short term profit. Yuji Horii on the other hand is making DQ games out of passion and wants them to succeed. Dragon Quest is his baby.
If he does say yes to a PS5 exclusivity offer for DQ XII, they will be lucky if it sells 2mil WW lifetime. 80% of the DQ fanbase is in Japan. DQ XI sold 2mil at launch in Japan on PS4 and 3DS.
80% of MH fanbase was in Japan as well. World sold more than 20 million copies still
We never know when a franchise can make a worldwide breaktrough. I'm not implying DQ will sell as much, but if they can sell let's say 6 to 7 million copies even losing half of JP sales is a good business for them, because it can solidy the IP internationally to sell even better in the next installment