For the scale/budget of their titles, ND are pretty productive. As to IP generation, the issue is that they are the originators of 2 franchises which are highly significant to the Playstation first-party line-up, so taking them off of those properties isn't easy from a quality control PoV. From a PR standpoint the new game would need to match or exceed the quality of previous entries so as not to be viewed as damaging to the brand, and frankly whoever takes over is inheriting a lot of pressure to achieve.
Transferring production to another studio also isn't ideal given that they are based on bespoke tech, meaning that they'd almost certainly need to be involved to some degree to hand over and support the new devs while they find their feet. This is basically how studios get kinda stuck with being single-property shops - success can really lock a team in.