True. They would necessarily have to add some new characters, which they did for both PIC & PRO, so not really a big deal there.
Yes, we've lost Eisenberg & Auberjonois but, as sad as those passings have been (and RDM's proposed storyboards in Ira Behr's DS9 documentary "What We Left Behind" notwithstanding), Nog's absence can be hand-waived away by his occupation in Starfleet (maybe even make him a Captain like they did in STO) - perhaps have him and his crew succumb to the whole transporter-virus thing they established in PIC S4, and Odo is permanently involved with the Great Link and continues to send intel back to Starfleet from the former Dominion (hinted at by Worf in PIC). Is it a somewhat macabre way of compensating for real-world events? Sure, but it's not an insurmountable task by any metric.
At the end of the day, yes, Brooks' very well-known disinterest in pursuing the Sisko character further is really the only main sticking point for a DS9 continuation. I guess, so long as they get permission from Brooks, they could potentially do some kind of AI-deep-fake kind of thing for him. The tech is there, but they would really need to think it through on that one.
Again, unlikely, but never impossible. If they're going to do it though, they better do it soon before more actors who played higher-level impactful characters also get too old or sadly pass on. None of them are getting any younger.