I wasn’t talking about data compatibility here, but rather about the compatibility of Base with the rest of the ecosystem from the user’s perspective. Since the main introduction to OCaml is now RWO via Base/Core, the more incompatibilities a user encounters as he/she meets the rest of the OCaml ecosystem, the harder it’ll be for said user to become comfortable with OCaml. It feels like we’ve taken one step forward with jbuilder becoming the de facto build system, and then one step back with introducing this new break with the rest of the language.
As one potential example, a user coming to OCaml from RWO may be completely unfamiliar with applying functors since they’re essentially unnecessary in the new Base world. I realize that RWO covers them, but they’re not required for daily use in Base and so may be skipped, similar to the chapter on objects.