The actual potency numbers themselves don't have any meaning in isolation because the damage formula itself changes from expansion to expansion. It's been a while since I've looked at this, but I remember in Stormblood there was a job-specific multiplier in translating potency to damage, such that you couldn't really compare potencies precisely on two different jobs.

The reason for the numbered upgrades ('Stone 7') is to keep the gameplay feeling fresh in terms of animations. It usually means that there aren't any fresh ideas to be added (Healers) but we're starting to see it more on melee and tank jobs simply because some of these actions are over 10 years old. And when you go from Stone 6 to Stone 7, players expect potency to go up.

The main reason why the numbers change, however, is because most job nerfs happen during expansion transitions. If they reduce your potency values now, there will be a massive outcry on the forums. If they do it between expansions at a time when nobody knows what the potencies actually mean (because the damage formula has changed), then you can't complain really.

Ever notice how jobs only have an 'upward balance' where you only see buffs go out in response to player demand? All that gets reset and rebalanced between expansions.