In past I sold my 35L as it was +10y old, with this money I bought the 35 Art (newer and better).
I'm a bit worried with the expensive 85L (MKI) I bought for 1100 USD as the value dropped a lot probably with version II.
AF would be a tad faster, less CA and some sort of advantage with the flash (ETTL or something).
The Sigma 85 1.4 isn't maybe a real competitor (not f1.2 and I heard the (onion?) bokeh was less nice and something else), but when the 85 Art will come out it will be a different story
135L & 70-200LII keep their value most.
In future I want to sell my 16-35 f4L IS for the future Laowa Zero-D 12mm f/2.8 http://photorumors.com ...on-free-lens-coming-soon/
I know this unknown brand will keep less it's value, but it's wider, no distortion, faster,....
The Tamron 24-70 2.8VC is a great standardzoom for everyone, this one I will probably sell for a 35 1.4 Art again.
I sell lenses when a new one is better in everything and when I don't lose (much money).
With a 'new one' I mean a new second hand lens, I only buy secondhand to avoid to pay too much extra.
In past I upgraded my second hand 5DII for a 6D MKI (new one from the store as it was not available secondhand yet) without having to pay much extra.
When the 6D II will come out I would probably need to sell an extra lens to have the same value of the new price, but I think I keep my 6D this time.