Icedove (Thunderbird) works well enough for me. Maybe the reason it’s “old fashioned” is because it works well enough that it doesn’t need to be changed that often.
In the proprietary software world we’re used to UI’s being redesigned on a regular basis for no user benefit.
Vivaldi’s target audience is people who don’t mind proprietary blobs as long as they are “good” or make things “work better.” Given that Vivaldi itself is essentially a proprietary blob combined with a Chromium backend this makes sense.