[Formendacil]

Sauron certainly neutralised Númenor as an enemy--but only insofar as he preyed upon their own desire for world supremacy. As long as Sauron was still their defeated captive and was pushing them towards a goal they long sought, he was able to get them to do what he wanted. Had he tried to push them back to war in Middle-earth though, it seems likely that the Númenóreans would have become suspicious he was trying to use them as a powerplay--not least because they had little interest in looking east: they'd already found and taken what they wanted there; the things they still wanted lay westward.

It's worth pointing out that we have no idea who Ar-Pharazôn's heir was. Amandil and Elendil were closer kin to him through his grandmother, Inzilbêth, than through their shared descent from Tar-Elendil. Presumably any one of the "King's Men" kings closer to Pharazôn had other descendants.

All things considered too, Sauron was much more interested in getting the Valar to act against Númenor than in destroying his enemies in Middle-earth--after all, he still had the Ring there and Númenor and Valinor were each comparatively greater than all of his Middle-earthbound enemies. I don't think he expected Valinor to ever get involved in Middle-earth again; he seems to have read their reluctance to enter the War of Wrath and their failure to pursue him after he left their custody as a sign they had given up, but I think he wasn't beyond spiting them--and maybe he wanted to use the Númenóreans as a test-case to see how far they could be pushed. No danger to him if they proved angry enough to sail east and wipe the island out.

What he didn't expect was Eru stepping in personally--and note that the consequence was that his body was destroyed and he had to rebuild it before the War of the Last Alliance. Granted, it took him far less time than it did AFTER that war, but it should be considered that the War of the Last Alliance would have gone MUCH differently if Númenor had been wiped out "conventionally" and Sauron been able to return to Middle-earth with body intact--and I think we can assume that this is what he was planning for.