Considering the MMOs they have to offer and their history, they would sooner revisit XI than go to a new product. It would cost significantly less to modernize and push XI than it would a new MMO currently even if said "modernization" is an essential soft relaunch. It is by no means cheap, but a ballpark figure based on the company I work it would virtually be choosing between 20m and 50m and that's not considering if SE would float the bill out of their own pocket or opt to build a new engine for a new MMO. Remember, XIV itself was only born out of the logic that it would have been wiser to make a new MMO at the time (2005) than revamp XI since there was nothing inherently wrong with XI in order to reboot it beyond graphically. It would have been double the revenue if you had players subbing to both or with XIV being easier to get into, you'd make more profit. XI is currently a windows only title now, the biggest hurdle is rewriting it to DX9/11 or DX12 first and foremost. (That isn't a hard thing to do but it still requires SE to invest in it.)
Making a new MMO of a new IP would be significantly more risky, that's why XI was a numbered entry to begin with and why XIV 2.0+ still has the "Final Fantasy" title even though the only thing that makes it FF are fanservice nods.
Remember - they tried doing a brand new MMO separate from XI and FMO, it didn't work out because it had nothing to stand on. That MMO was called Fantasy Earth. It wasn't terrible but it wasn't "Final Fantasy", which is why "Final Fantasy XVI" would sooner be an MMO before one based on any other IP or brand new IP as DQ already has DQX. Reason I say they'd revisit XI is because it was said a few times if they had the team and funding there's a lot more they can do with it. Hell the new UI they were working on for XI is in limbo because of that reason.
Once XIV became their baby every other project has been kind of put on the backbunner. Including DQX.