Also, kernel upgrades. Unless the user knows about and specifically opts to use Debian backports, they’re going to be on the same kernel version until the next stable Debian release. It’s not the end of the world to leave performance on the table, but some people are picky about getting their money out of their hardware.
Using backports and upgrading to a newer kernel is fine for someone familiar with Linux and confident enough to tinker and make at-your-own-risk changes. Having to do that can be offputting for newcomers, coming across as intimidating or unnecessarily complicated.
It’s not so much the lack of a rapid update cycle as much as it’s the pinned kernel version alongside the years-long pace of Debian’s stable upgrade cycle.
That would be fine if the kernel didn’t see much improvement over ~2 years of development, but there’s constantly new stuff being added or optimized with every kernel release. It’s just not much of a friendly introduction to Linux gaming for a newcomer to either have to pick between missing out on recent improvements, or diving into the intimidating realm of fiddling with packages and backported kernels—especially if they’re not coming from a tech savvy background.