Personally I prefer shop=plumbing_supplies - while shop=plumbing is used more it can be easily confused with places providing plumbing services, or office of company doing this.
I don’t think there’s any ambiguity about shop=plumbing and it has 6 times as many uses as the next candidate. So, although plumbing_supplies may offer a small bit of additional precision I see little reason to change it. Note to my ear plumbing_supply is more ambiguous than the other alternatives. There are, or more accurately, were quite large chains dedicated to this sort of stuff (the former Plumb Center, although in practice I found them fairly useless for one component I wanted a few years ago and went off to a chain D-i-Y store).
I would think shorter tags were favoured before editors had a large set of presets. There are numerous other shop=* tags which could be altered in the same way, but are well-established and mainly hidden from contributors in most editors. Similarly craft=* is inconsistent in usage, with some tags being obvious nouns and others nouns orthographically identical to intransitive verbs, as here. Without a consistent formulation of how to construct suitable tag values ad hoc changes will tend to be more about personal opinions than a systematic approach, which is more or less infeasible now.
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