Here is some mostly good news: Microsoft developers can now get perpetual dev tenants for Office 365.
Previously, dev tenants were good for one year, and then expired. According to this FAQ, as of April 2019, tenants are good for a minimum of 90 days, but will continue as long as they are in use.
The only downside is that this does not apply to legacy tenants. I created a dev tenant last December, but unfortunately, it will expired December of this year.
At least I have a few months to build out a new dev tenant to replace it.