Lambda@Edge unfortunately does not support provisioned concurrency, so you are bound to cold-starts as well (I tried, measured, and reverted before even going live with it).
Running something locally is not necessarily free either. Even if maybe the "host" is for free (which I can not imagine, unless someone throw something at you), it will still require your time to setup and ensure its running.
For private/small open source projects I would highly welcome such a pricing tier from travis-ci but 69USD is just not in my pocket (not saying its unjustified though!).
For larger open-source projects that rely on many builds throughout the day/month, this might be a bigger problem.
So while its "free" marketing, it might be much more crucial to our open-source structure than we think it is (which might be a problem as well).
And I think we can all agree: open source software is an important part not only for the industry but our lives :)