Reducing dependencies has a lot of benefits, even outside of build failures. I think that can be separated from the suggestion here though.
If your suggestion was the default, it would still be up to everyone to “configure their version control correctly” such that they are (1) using version control in the first place and (2) committing this directory and managing changes through that system.
So the suggestion here also relies on recommendations and “correct configuration” to work well, but I suspect far fewer people would “correctly configure” their system like this. Putting aside people who wouldn’t want to do this through version control, many students in high school and college are not using version control at all. I was not at least! From there, “misconfigured” systems would result in higher volume of requests to various services since downloads are per-project.
It looks like you can experiment with ELM_HOME to see if the approach you suggest works well for you. If so, perhaps it would be useful to compare to caching ~/.elm so others can better understand the tradeoffs that you faced when evaluating different options.