"Add-on" is the keyword here. Instead of disrupting your workflow by offering a distinct native app developed from scratch, this simply enhances an already great experience. I consider Gmail's UI very efficient and actually quite close to perfection (speed, reliability, labels, filters, multiple inboxes, keyboard shortcuts...). Gmelius just corrects the small annoyances that remain or have appeared lately.
This reminds me of a sentence in Dropbox's YC application: "With Dropbox, you hit "Save", as you normally would, and everything just works." The worflow isn't altered. It's just better.
Of course, Gmelius is just and add-on, with no clear business model. But maybe that's all we need after all.