The thing is, the "more nuanced" view was taught consistently before the Council as well--it was really nothing new. While the underlying idea that it was possible for even non-Catholics to be saved given certain circumstances goes all the way back to the Fathers, it was first really explained in detail around the 16th century. Both large swaths of people being led away into Protestantism and the discovery in the new world of huge populations of people where the Gospel was never promulgated led to this development. This is when St. Robert Bellarmine developed the terminology of belonging to the Church "in voto," as compared to being an actual member, for example.
From then, all the old theological manuals and catechisms right up to Vatican II affirm the same thing.
Likewise, the idea that people born into heresies are themselves not to be presumed guilty of heresy is also not new. It is found in the writings of Cardinal Manning concerning Anglicans (Manning is historically contrasted to his contemporary Cardinal Newman as being much more conservative than Newman on such matters).
This idea itself goes back at least to St. Augustine, who noted:
CHURCH FATHERS: Letter 43 (St. Augustine)
St. Augustine also taught the idea of partial communion:
In the Treatise which we Wrote against the Published Epistle of Parmenianus To...
These ideas didn't get rid of evangelical zeal or fear of heresy before, so why now? That's why I think it must be something different than just the repetition of this idea by the Council.