They have occasional landing failures (one this year, two last year), and still do launch expendably every once in a blue moon, so they'll likely need to build a new one every once in a while until the end of the program is in sight.

(It looks like they have very few customers anymore that insist on a new-build booster -- the fussiest had been the US government, but at this point, NASA is accepting reused boosters for human spaceflight, and the NRO has also accepted one for a mission last year. As far as I know, no one has publicly said they'd rather their booster has a test flight yet.)