I'm using tsx for a project to achieve the same effect. As you said, it saves you from having to set up a build/transpilation step, which is very useful for development. Tsx has a --watch feature built in as well, which allows me to run a server from the typescript source files and automatically restart on changes. Maybe with nodemon and this new node improvement this can now done without tsx.

To check types at runtime (if that can even be done in a useful way?) it would have to be built into v8, and I suppose that would be a whole rewrite.

Node has had a built-in --watch flag for a while too:

https://nodejs.org/docs/latest/api/cli.html#--watch