Of course its back projection.

But it's also backed up by observation of what's on screen (which is what I
said -- "my own observation"). In things like 'Hide & Q' and 'Too Short A Season', we already see Worf in roles that might be
expected to belong to the security chief. Crosby herself said
on tape and on the record that she was annoyed at the scripts not "showing us what this so-called female chief of security can really do". It's supposition on my part that she saw these things happening herself
at the time, but it also isn't much of a stretch based on the evidence. Nobody knows for sure except her (and I suspect by this stage she's hardly going to come out and tell us.)
We might never know how the show may have progressed with both Worf
and Yar in the cast, especially after the writing team finally started getting their act together from the second season onwards. Scenes like their interactions in '01001001' and 'Skin of Evil' seem to suggest an interesting friendship/dynamic between them. But it feels instinctively like one character kind of pushes the other out of the picture, unless they'd been willing to adjust Tasha's role accordingly. It's also interesting that the episode where they brought her back ('Yesterday's Enterprise'), it's in a universe where Worf isn't on the Enterprise to get in her way.
Alternatively, given the producer's willingness to play with the cast line-up and drop characters between seasons (Dr. Crusher), it's entirely plausible that if Crosby had stayed around, she may have suffered the same fate: being written out between seasons and with Worf simply transplanted directly in her former role. At least the way things went, she got a departure on screen, and became a permanent footnote in Star Trek history to boot as the first regular character to be killed off and not brought back. I'd call that a good legacy, considering the alternative.
