I’ll use ATM’s two examples and add what I think makes the two scenarios somewhat different.

Cushing;
"…been dead for 20 years" - and in-universe has been dead for 30 years, in a film released 40 years ago. The film currently in the theaters tells a story that ends moments before the 1977 film starts.

Fisher;
"…died unexpectedly young" - and in-universe is part of the current story, which she just finished filming for a release this year. The film next year is also part of the current timeline.

Cushing; They either had to recreate Tarkin, CGI or otherwise, or have a strange a absense in the current film & story.

Fisher; They can do a Col. Blake and have the story work very well. It would also help the fans, who at this point only a month since, are still making peace with her unexpected passing. This is how I hope they handle it.