Hi Legacy. You might try the Salvation Army, St. Vincent DePaul, and Goodwill Thrift stores, as they always have a few NWTs in their second hand book sections, some in mint condition. Of course you could follow your own advice and call Watchtower headquarters and ask a staff member why they let the English NWT go out of print.
Regarding a revised NWT, I think Slimfatboy is right on. The foreign language NWTs are not translated from the original languages like most foreign language Bibles, but are translated from the English NWT. If the Watchtower is too cheap to employ competent scholars to translate their foreign language NWTs from the original Hebrew and Greek, then they won't go to the expense of revising the non-English NWTs to agree with a revised NWT English version. And since the Borg requires conformity, no revision on the horizon.
Since we're speculating about why the NWT is out of stock, here's another possibility. Freddy Franz completed the NWT in 1960, and with minor revisions, the JW Bible has kept Freddy's peculiar renderings and late 19th Century wording. In fact some of the language and terminology of the NWT was obsolete at the time it was first published. So an NWT in modern English is needed if the JWs want to understand their own Bible.
However, 40 years ago the Watchtower released THE BIBLE IN LIVING ENGLISH, translated by Stephen Byington. The BLE was a much better translation than the NWT, but do to an agreement with Byington's estate, the Watchtower wasn't allowed to edit Byington's work to fit JW beliefs. But what if the Watchtower acquired a manuscript similar to Byington's work that was written in 21st Century English, but with the proviso that the Borg could make adjustments to the translation to fit the JW theological bias? This solves two of the Borg’s problems.
1. All JWs will need to donate to obtain a copy of this new Bible. The cash flow improves.
2. Like other churches, the JWs now have a modern English Bible that they can sort of understand.