Joel said:
The O-Neg is the intended finished product, not an unfinished product waiting to be somehow "corrected" by generational loss.
Doesn't the O- have none of the SPFX?
Furthermore, directors work under the assumption that some things will be hidden due to generational loss, so I'd hardly call the O- the "intended" finished product.
To make an analogy, HD audio restorations often don't go all the way back to original multi-track master tapes because that would require remixing rather than remastering, which can fundamentally change the music (e.g., the 40th Anniversary Edition of Jethro Tull's Aqualung).
If the SPFX isn't on the O-, then cobbling together a restoration from the O- and various intermediate print sources containing the SPFX would be similar to going back to the original multi-track master tapes for an HD audio restoration, which is a big no-no.