Nightblood can replicate the effects of 9 out of 10 surges now. And while radiants with access to progression are functionally useless outside of locked-down Urithiru where there's no access to Stormlight, cutting people like Adolin off from regrowth, Nightblood can consume any form of investiture, including potentially abundant warlight. It shouldn't be hard for Szeth to get his hands on some spheres (though maybe he'd have to buy them from the Listeners? Maybe not? Maybe he can get some in Shinovar?) and work with Nightblood, who's gotten a big leg-up in the whole self-control thing. Losing his arm seems like it should be only a minor inconvenience for him.