I think it is unfortunate that so many digital audio products offer firmware updates so often that it's confusing most consumers to expect regular firmware updates.
To me, the firmware/software of DACs can really only do a few things:
1) Change the upsampling algorithm
2) Change the noise-shaping algorithm
3) Change the USB interface/Ethernet interface
The problem here is that the hardware of digital audio products are fixed so
1) The upsampling algorithm is always limited by the computational processing power of the hardware built into the product. So ideally, you have optimized and maximized your upsampling algorithm and get optimal sound. To ask why there aren't new firmware to improve the sound, is like asking why isn't there a software update for my Apple II computer from 40 years ago so that it runs like a new M1 Pro MacBook Pro?
2) The noise-shaping algorithm that is tailored to the DAC architecture is also limited by the computational processing power of the hardware built into the product. So once again, ideally, the company should have optimized the noise-shaping algorithm at the time of product release so no further sonic upgrade would be possible.
3) Upgrading the USB interface definitely might improve the sound as a new firmware might run more efficiently and draw less power leading to less noise in the rest of the DAC circuit or less jitter overall. But USB interfaces would also be influenced by the music server that you're feeding the USB interface. So I can see how companies can sometimes change the code once the product is released. This applies less so for Ethernet interface and other feature upgrades.
Ultimately, my take is that from a sonic quality side, a DAC once designed, shouldn't "need" further firmware updates to improve the sound. Often companies that offer such upgrades were not optimally using their existing hardware built into the DAC and that's why they're able to provide these periodic updates.