After the 60-Day free trial, Microsoft may offer you an option to extend the trial. Power BI is their service. The only item provided by Maven is the work email that you can use to create a Power BI account. Other than that, we have no influence on the Terms and Conditions of Microsoft.
However:
Your old projects will always be visible in Maven Showcase which is available with our Starter plan (free).
You can always share your new Power BI reports from Power BI Service via "My Workspace". This feature will remain available to you even after your trial ends (included in a free PBI plan).