However the existing operator could do this today.
That's always been true of a lot of NR metro area operators though. TfL have proven their ability to quickly go in and do some easy wins that the NR TOCs have had decades to do, but didn't. For example the North London Line and the GEML and Lee Valley metro services are far improved, within only a few years of TfL operations.
Meanwhile TOCs, like Southern and Southeastern in particular, still have plenty they could do to improve the metro services, but don't. So although yes "the existing operator could do this today" , they have had decades to do so and haven't - if TfL are prepared to do so, I think let them try.
Admittedly - and I say this even as a proponent of devolution of metro services to TfL - it is less clear on the Great Northern Inners what improvements TfL could actually make.
Upgrading the rolling stock to 717s was a fantastic improvement and was imo the easiest win on the line - already done by GTR. Station refurbishments and improvements maybe?
But in terms of improving the train service, separating the Inners from the rest of GN may prove to be difficult or more trouble than it's worth, I honestly don't know enough to know.
But, it was clearly possible to do so when Overground gained South London Line, Lea Valley, and GEML metro services away from existing TOCs that otherwise continued to operate.
(Silverlink was a bit different as the entire North London+GOBLIN operation was already quite segregated so I believe easier for TfL to annex, though someone like
@ChiefPlanner will have better knowledge of that and how it compares to GN now.)