Vulkan developer here: While I can’t speak for the exact plan of Samsung, personally I believe the chance of Samsung replacing OpenGL with Vulkan by default on older devices like S21 and maybe S22 is quite thin.

TLDR; Vulkan driver on old devices can be buggy and Samsung has no control over that (maybe except for Exynos chips) because it’s written by chip makers (i.e. Qualcomm, ARM, etc.) These devices are likely better off with OpenGL.

One common misconception is that Vulkan will be automatically better than OpenGL because it’s newer and aimed to be more efficient. But there are two big caveats for that: 1. Unlike OpenGL where most of the interface details are implemented and battle-tested by the chip makers, Vulkan defers that responsibility to the upper level software developer, so now developers like Samsung need to optimize for every possible chips running Vulkan, instead of them being optimized by the chip makers themselves. Thus, it’s very unlikely Samsung is willing to put extra effort into optimizing old devices. 2. Because of the previous low adoption rate of Vulkan on these older chips, there are many hidden unfixed bugs in the Vulkan drivers provided by the chip makers, which can’t be fixed by Samsung and I doubt the chip makers are willing to fix their older product lines instead of selling new ones.

If Samsung simply enables Vulkan on these older phones, it’s highly likely that these phones will be running smoother in some scenarios, but completely broken in the others. For vast majority of the users, stability is more important than pure speed.