Precisely.
People tend to have Issues when even
one single character gets recast. Consider the issues with T'Challa needing to be recast because of the unfortunate death of Chadwick Boseman, the original actor. They literally put in an explicit "This actor is Black Panther now" scene with the new actor (in context, having a minor beef with Tony Stark and saying "I'm here, deal with it" or something like that,
nudge nudge wink wink say no more, say no more etc.)
To lose half the cast every movie--such that none of the same characters are still there by the end of the third--would pretty much kill any franchise ever.
Continuity matters, and people really do care about
both specific-character continuity AND intra-party continuity.
Because,
@Lanefan, while you are correct that the group is more than the sum of its parts, when you literally remove
the entire sum of the parts, most of the things that were "more" than that sum also disappear. Most of that "more than the sum of the parts" is in the many different interrelations between specific people.
If we have a five-member party, for example, you have the five individual characters, sure. But you also have
ten pairs (AB, AC, AD, AE, BC, BD, BE, CD, CE, DE), and ten more triplets, and five more one-person-missing sets.
All of those things can be interesting in themselves, and they are--necessarily--more than JUST the sum of A + B + C + D + E as individuals.
People, being people, tend to attach to people rather than vague, nebulous, ill-defined group notions. Kill off everyone they liked--and thus every single possible intra-group dynamic--and people will tend to lose interest very quickly.
The exact same thing happens to a lot of people, I'd argue most people, when you have serial loss of characters in a TTRPG. Sure, they can
choose to try to keep up their investment despite having it thrown away over and over, but there's only so much investment most of us can give before we just become numb and
stop caring.
Remember that the eight deadly words happen both when the story fails to actually grip us,
and when we aren't given enough reason to keep investing. Those eight deadly words are: "
I don't care what happens to these people."