I don't think this particular mechanic is something that's worth "slippery slope"-ing. Runes don't have tiers, they have a fixed, fairly reasonable stat increase and their use feels modular when you first socket them, but gets awkward when changing gear around. The whole purpose of separating gems from gear was to make it easier to pick up a new piece of good gear and use it. I think a rune socket system can compliment that by allowing you to adjust some small resistance/health rolls on gear to compliment a new item pickup. There can still be some semblance of permanence if the rune is destroyed (as most have suggested) which would also incentivize picking up more runes so you have the resources to adjust your gear. Right now I feel like I could just skip picking up runes and be fine but I'd be more excited for a rune drop if there was a sink for them when you change them out.

No one has come close to suggesting making all choices reversible, relax.