If someone can spend 5L in a year, he would get DCB instead. For less than that, Millenia gives more value.
You need to convert regalia reward points to actual value, as flights have discounts from 15-20% and you need to pay 30% from card while redeeming.
5x reward points from regalia is much less than 5% cashback from Millenia for instant vouchers, when u consider those flight redemption factors.