My main concern is stability. Changes to any of the ISO 639 family have the potential to have a very negative impact on worldwide IT. So it would be better to leave everything as is, than to risk that level of disruption.
The survey does not capture the usage of ISO 639 with such questions as:
2. Please indicate which parts of ISO 639 you use, and how frequently:
Part 1Part 2Part 3Part 4Part 5Part 6
The point is not whether I personally—or anyone else—is "looking at" the standard. The point is how many users are affected by changes. And that is a huge number: every user of a smartphone; every user of a web browsers; every user of a search engine, etc.
Any proposed "unification" of the standards or of the registration needs to weigh the potential costs and benefits. For example, a merger of the standards would only be acceptable if it was purely editorial; but in that case, it is hardly worth the effort. A merger of the Registration Authority is likewise only warned if it does not disrupt the stability and established practice for adding codes; otherwise it would be a disaster.