Lowly gold elite member here and I received that same email about the Tribeca Sheraton closing. I called the reservation line and told them I was not interested in being transferred to a Hyatt Place and that I wanted to stay in the Marriott eco system. The rep told me she is unable to transfer me to a comparable Marriot hotel (i.e., another category 6).

She said she would email the Sheraton Tribeca and ask if they can transfer me to another category 6 hotel, but I am not going to hold my breath.