Responding on Twitter today, Senior Game Producer Josh Greenfield acknowledged that the elusive Rune of Metamorphosis is an outlier, pledging to make it easier to obtain in future phases of WoW Classic Season of Discovery.
For those unafraid of spoilers, the recently discovered Rune of Metamorphosis was one of the last found in Season of Discovery, as it's acquisition begins with defeating the first Blackfathom Deeps raid boss, whereas the majority of other runes are obtainable much earlier in the Phase 1 leveling process. While reaching level 25 and finding a group for the pinnacle activity of Phase 1 may not be a significant hurdle compared to many achievements within the game, it is a much greater investment than other runes and somewhat odd that the entire Warlock tank role can't be unlocked until having already progressed into the main activity it would be used for.
With future phases increasing the level cap beyond level 25, it does seem likely that Blackfathom Deeps will be harder to find groups for, as its rewards will quickly be outscaled by new quest, profession, and dungeon gear, which could prove a hurdle to new potential Warlock tanks. Adjusting or adding an alternate way to acquire the rune would therefore be a welcome quality of life improvement.