The concerns about Wunderpus are three-fold, and many questions remain before we can assess if/how rare/threatened Wunderpus is in the wild.
1) Population status: In some places they seem exceedingly rare, and they live in a highly threatened habitat. Not being as sexy as coral reefs, tropical soft- sediment communities are largely ignored in many tropical conservation initiatives (though many of us here are working on that!), and still trawled heavily in many places.
However- like you, I have seen sites of high population density. This begs the question- are particular areas good as nursery grounds, mating grounds, or is there strong seasonality in their reproductive activity? If any of these are true, then (as we know from many many other fisheries) it is dangerous to their populations to harvest at these times/places, because they represent a concentration of animals grouped from (or to disperse to) a large area.
2) In some areas there's strong reason to think that collecting them for the aquarium trade is in direct conflict with the dive industry. There is strong sentiment that a few people are gaining money from taking animals which could benefit way more people over a longer time if it were alive in its natural habitat.
3) Wunderpus have low fecundity- they produce very small batches of eggs and have low reproductive potential.
I think we have a long way to go before any of these issues can be resolved. If you know of a site where they are abundant that's fantastic. It would be great if that population could be studied to help get at some of these issues. Even just keeping track of the size and sex ratios of the individuals that come into your store throughout the year would be a huge help. Would you be interested in keeping track of these data?
If it turns out that Wunderpus populations are not threatened, and their harvests for the aquarium trade can be managed sustainably, then these animals have the potential to be a flagship species for responsible high-value alternative livelihoods and yet another reason to protect soft-sediments.