Thanks for the explanation. Now I see that my question came simply from the misinterpreted aptitude's suggestion to remove ifupdown and install ifupdown2. I made a mistake to assume that ifupdown2 is intended to replace ifupdown, and to think that successful install of ifupdown2 (which is not in conflict with current systemd) should make it work.
I'll wait for the new systemd, than I'll check if ifupdown2 also works with it, and go back to ifupdown.
