You don't have to. You can have write a single component in ReScript and use it with the rest of the project written in JS or TS. See https://rescript-lang.org/docs/gentype/latest/introduction

Sorry but that's not true. You can adopt ReScript one file at a time. ReScript supports importing and exporting typescript types out of the box so you still get the type safety from "the other side"

We started using Rescript by adopting individual files until most of our codebase is ReScript. We still have Typescript lingering around but folks _choose_ to refactor it to Rescript because they've found it easier to work with.