You are right about incompatibility being a major issue; Deno recognizes that as well, hence, they are working on a compatibility mode that allows using Node specific libraries in Deno[2].
> migrating to Deno is, charitably, likely a monumental task that nobody would ever undertake, even considering the upsides.
This is, of course, contingent on the architecture used: for code tightly coupled to frameworks/runtimes it is indeed a monumental task. I have two small to mid size SaaS apps happily running on Node.js, but I'm looking forward to replacing it with Deno solely for the streamlined DX. The apps follow DDD architecture, thus, framework specific stuff is decoupled into a service/adapter and changing it is a day's work. The major technical road-block for now is indeed incompatibility of third-party libraries/SDKs written for Node.js (google sdk, mongdbo driver, etc.).
[1] https://itnext.io/moving-libraries-to-deno-the-whys-and-hows...