See this FAQ:
https://blog.mozilla.org/addons/2019/09/03/mozillas-manifest...
Note how they say: "We have no immediate plans to remove blocking webRequest and are working with add-on developers to gain a better understanding of how they use the APIs in question to help determine how to best support them."
"No immediate plans" is weasel-speak, as is "[we] are working with add-on developers".
If Mozilla was dead sure that they weren't going ahead with it, they would say so, unequivocally. And I remind you that Mozilla "worked with add-on developers" when they unilaterally decided to drop XUL and go ahead with web extensions, while failing to include support for APIs that developers said they needed to support functionality that their add-ons provided.
IMHO, adoption of manifest v3 is not a matter of "if" but rather "when".