I will agree the reason solo content gets nerfed shouldn’t be M+ or raids, especially when the solo content is isolated from grouped content.
But it’s just an oversimplification to pretend like solo content when it has an impact on other content should never be nerfed. These things don’t happen in a vacuum. Personally I don’t think delves should be nerfed because I think the real reason for the disparity is an earlier change to M+; but that doesn’t mean there would never be a particular structure of solo content that has a negative impact on aspects of the game that shouldn’t be addressed just because it’s solo content.