That's an interesting idea, but it has some questions to resolve:
- what if some of the source files are available, and others are not?
- should we surface an error somewhere?
- won't this confuse users which, for example, forgot to launch the local server with sources? It would look like a sourcemap doesn't work in this case.

> what if some of the source files are available, and others are not?

Optimal would be trying to load the original source file and skip only the matching of these. But probably it is much easier to skip the mapping of the whole compiled JS file in this case. 

> should we surface an error somewhere?

Perhaps as a tooltip of the "non moved" breakpoint of the compiled JS file. 
And/or in the Breakpoint list on the right. A Title of a non working original source file could be strikethrough and/or red and a descriptive tooltip.

> won't this confuse users which, for example, forgot to launch the local server with sources? It would look like a sourcemap doesn't work in this case.

A tooltip could help in this case. 

An easy workaround would be to toggle the "javascript sourcemaps" checkbox in devtools settings. The toggle now happens live, no need to reload or break debugging session.