Thanks Squig and Deadcode for your replies.
After a while I decide to keep my 5D3s instead of switching to BMPCC4K. I'll share my thoughts here, hoping it could be useful for someone else in my same conditions (and to clarify myself too).
Giving that low-light and noise performances are roughly equivalent on both cameras, for my way of working (and for the fact that I'm a generic artist, not a specific pro) I see 3 PROs and 3 CONs in switching from 5D3 to BMPCC4K:
PROs:1. Weight and size: BMPCC4K is smaller and lighter than 5D3, its 5" screen can (often) replace an external monitor, and very-fast mFT lenses (that give same speed performance than SpeedBooster+FF-lenses) are compact and light as well. That means having at least 1 Kg less in your hands, and 2-to-3 Kg less in your bag (and this is not secondary for who has a good arthrosis

2. B-RAW: honestly MLV/CDNG workflow is a little nightmare: time consuming, storage consuming (with the price of my second RAID I could buy a complete set based on BMPCC4K!) B-RAW means having a quality-effective 3:1 file that requires 1/3 of the storage, can be directly drag into Davinci and is fast;
3. Dynamic Range: obviously improved over the 5D3 (never been a DR champion in comparison with other DSLRs)
CONs:1. Selling 2 cameras to buy only one: in order to take the complete advantages of mFT set compactness and lightness – so, to buy BMPCC4K + mFT lens-set – I should sell both my two 5D3s and all my FF lenses. That means working with one single camera instead of two! And I must say that working with 2 cameras is a great thing in various situations;
2. 5D3 is a very good still camera: that means huge timelapse quality! And as I wrote above, being a generic artist means that my working "paradigm" depends on each different project and can strongly change from time to time. So having a camera that can also take good stills is a big advantage;
3. MagicLantern controls: it's not secondary that ML gives you a bunch of very detailed controls (and functions: see crop mode...) that lack on BMPCC4K. Obviously you can do the job anyway, but things like the spirit level or the customizable cropmarks helped my work really a lot. In order to minimize this lacks on BMPCC4K, I could buy a full featured external monitor, but it starts to go in the opposite direction than "cutting ballast"...
FINAL THOUGHTS:All 3 PROs represent improvements, that means BMPCC4K and 5D3 do the same things but BMPCC4K can do them a bit better/easy.
Instead all 3 CONs are lacks! That means 5D3 can do things that BMPCC4K can't do!
THIS is the answer. That's why I remain with my 5D3s.
Eventually, I could opt for a compromise: selling only one 5D3 (and adding 1K€ more...) in order to buy the BMPCC4K body-only (no mFT lenses) + SpeedBooster. In this way I could keep one 5D3 + one BMPCC4K and sharing le FF lenses between the two. But I remember that I bought my second 5D3 (with many sacrifices) just because it was discontinued, and once my first camera broke down, I would have lost the best/only camera for ML. So, again, for the moment I'll keep my 5D3s
