Upload & Sell: Off
If you (i) like the 15mm a lot, which you do; and (ii) shoot a lot of nature where high impact deep scenes is desirable, which you may, then the 21mm is a must-have lens, I feel. I confess I don't see many images of that kind in the threads here from the 21mm. People seem to not enjoy the focal length, and many gravitate towards the 24/25mm area. It would seen to suit what you do, however.
If a 15mm floats the boat, 25mm is too much longer, remember even 2mm is very noticeable for super wides. It's hard to think of another lens that epitomises the Distagon genes more than the 21mm, and a special note of the film era version I have, which is more dreamy and yet sharper at f5.6-f8 than the ZE - which is clearly optimised for wide open use, a somewhat different lens. It (the ZE) is more in keeping with the new wide open use policy CZ uses for even its wide lenses nowadays. I like the old Contax line - film era preference.
Many people, I am sure think to themselves re the 21mm: 'what is all the fuss about?' and they should look elsewhere for satisfaction, perhaps the Leica 28mm v2 as well.
But I can't imagine working without it, and still stare at the large jpegs on my monitor at work. I see a lot in common in the 15mm and would aim to get one when I get back to more landcapes, it is a bit big and marginal for travel, for me anyway.
I am finding that 25-30mm a 'dead spot' and have very little trouble making the jump to 21mm from the wide end of (the medium wide to medium long) 35-70 FL range for 'normal' perspective shots. SO there is another view.