Why not? Even without LLMs it is technically feasible to build custom database engine that performs much better than general database kernels. And we see this happening all the time, with timeseries, BLOBs, documents, OLTP, OLAP, logging etc.

The catch is obviously that the development is way too expensive and that it takes a lot of technical capability which isn't really all that common. The novelty which this paper presents is that these two barriers might have come to an end - we can use LLMs and agents to build custom database engines for ourselves™ and our™ specific workloads, very quickly and for a tiny fraction of development price.