I wonder if keeping empty sexplib.syntax subpackage with only requires="pa_sexp_conv" line
will prevent build breakage (one will only need to fix opam dependencies)?
That would work for now. However, sooner or latter some change in
sexplib will break the code generated by pa_sexp_conv and we'll have the same problem again.
The current forced upgrade is simple (one sed command [1]) and it's a hint that it's a good time to switch to ppx. We published a tool to help with this BTW [2]