I actually wouldn't be surprised if we don't see anymore DS9 novels (or Voyager novels after To Lose the Earth comes out eventually).
I think with all the shows CBS will have on the air soon, Pocketbooks is going to focus on tie ins to those shows, and original series novels will always be released. And I think TNG novels will continue until the nu-TNG show comes out.
But assuming the nu-TNG comes out and wipes out most of the current litverse (which I expect it will do though they may cherry pick some elements to survive) I really doubt they'll commission any more DS9 or Voyager novels (or probably even Enterprise though that is less likely to be affected).
And part of that is marketing. They're in the business to sell money and while it saddens me to say this a Discovery, nu-TNG, or Section 31 show based novel is more likely to sell books than a book tied in to a show that ended years ago. It was different when there was no shows on the air and just 3 Abramsverse movies, but now it will all be centered around the current shows---and again original series novels (those will always exist).
I think you're probably right. It's a very different environment now, both in publishing in general and Star Trek in particular. There are only 12 months in the year (and what we've seen of the 2019 schedule suggests S&S is publishing on a not-quite-monthly schedule), there are tie-ins for the new series to publish (and will be more in the future), and TOS and TNG are probably never, ever going away (though I wouldn't be surprised if TNG novels, at some point, move to during the television series). There's probably not room on the schedule for the original-to-novels series like New Frontier and Seekers (and maybe even Titan), and if there's a choice between a Deep Space Nine novel and a TOS novel, there's more money to be made on the TOS novel. It wouldn't surprise me if S&S's current contract, like IDW's comics contract, doesn't even cover all of the branches of the franchise; why pay for the rights to stuff they're not going to use? This is all speculation -- less than a book a month, only some of the series -- but it strikes me as quite possible, even likely.