I've been using Simplify (v1) for so long now that when I do come across the original Gmail design I find it visually jarring and overwhelming.
This is such a good product and I'm glad that it's moving into a paid for business model because I can give them my cash in the safe knowledge that the extension will continue to be supported even as Gmail changes under it.
Free is great, but longevity is better and always worth paying for.
Source: on.simpl.fyi