So while most of the software is open source rather than proprietary, you still have a fair point that customers pay for support (as they do with most enterprise products). One could theoretically use the product without first-party software updates, managing the open source oneself... but that would have practical impediments (and runs counter to the all-in-one simplicity that customers value in the Oxide product).
Two points about your last point. First, software improvements benefit all customers; as the business grows, the effective cost per customer shrinks. Also, most customers grow their Oxide deployment or will replace hardware after a depreciation cycle. The sustainability of investments into the software (and the product generally) is on solid ground.
Oxide uses illumos under the covers That's in no small part due to the composition of the early team--but we have benefitted tremendously from features (ZFS, SMF, FMA, Zones, bhyve, etc.) and observability infrastructure (DTrace, mdb, libproc, etc.) folks on the team have worked on for decades.
Two points about your last point. First, software improvements benefit all customers; as the business grows, the effective cost per customer shrinks. Also, most customers grow their Oxide deployment or will replace hardware after a depreciation cycle. The sustainability of investments into the software (and the product generally) is on solid ground.