I was writing from the aspect of an Lisp implementer, so that means it is not how I can just use vectors but about how existing code represents a sequence, and it is part of the job of the runtime to make it as fast as possible.
From what I see from existing ELisp code (at least in the Emacs codebase), the idiomatic representation of sequences (fixed-size or not) is using cons lists. And it is not surprising: Emacs vectors are fixed-size and that makes it very inflexible and only suitable for a few things. This matters because, for example, if you want Emacs to compete with VSCode in performance, you eventually compares how idiomatic code performs. Note that how cons lists affect performance in real-world ELisp code remains unknown because it is yet to be benchmarked, but exposing the internals of idiomatic lists as conses does pose some challenges for an implementer aiming for further optimizations.
Emacs Lisp vectors being fixed seems like an easily fixable problem. More functions can be defined to do useful things with vectors including mutations. If it is important to keep the existing type produced by make-vector immutable, a separate mutable variant can be introduced. The mutating functions blow up if applied to the immutable type.
From what I see from existing ELisp code (at least in the Emacs codebase), the idiomatic representation of sequences (fixed-size or not) is using cons lists. And it is not surprising: Emacs vectors are fixed-size and that makes it very inflexible and only suitable for a few things. This matters because, for example, if you want Emacs to compete with VSCode in performance, you eventually compares how idiomatic code performs. Note that how cons lists affect performance in real-world ELisp code remains unknown because it is yet to be benchmarked, but exposing the internals of idiomatic lists as conses does pose some challenges for an implementer aiming for further optimizations.