I wonder why they very carefully avoid giving a hard minimum for how long the software will actually be supported. "For years to come" could be a third of your device's physical lifetime like everyone else does.
Maybe it's because the phone's just running Linux. I believe [1] they have upstreamed a lot of their work into the kernel and Gnome so really as long as those projects keep going it should be fine.
1. https://puri.sm/posts/librem5-2018-11-hardware-report/