The excerpt from the Bisnow article sums up nicely how Marriott is using the Delta Hotels brand as a conversion brand, and why: Marriott is emulating Hilton's success with DoubleTree.

I don't mind the idea of a Marriott-affiliated full-service hotel that previously flew a different flag. If the hotel has been renovated with modern furniture, new flooring, cosmetic changes in the bathroom, clean decor, and a pleasant lobby, it doesn't need to adhere to all the specifications of a new-build full-service hotel. This should fill a niche between higher-end full-service hotels such as Renaissance and select service hotels such as Courtyard.

However I have two concerns about Delta Hotels:

It seems many of these hotels were in sad shape before getting their Delta renovations. I hope Marriott will monitor these properties carefully to make sure that standards don't slip.

I've read about the self-service Elite Pantries at some Delta Hotels properties. These take the place of a real Signature Lounge or a real breakfast in the restaurant. Considering that Delta is treated like Marriott, Renaissance, JW Marriott, and Autograph Collection in the terms & conditions, the Elite Pantry seems inadequate (even if it is mentioned as an approved alternative in the terms & conditions).