I don't mind having my legroom restricted with my stuff but I wasn't going to have it restricted with other peoples. Would the cabin crew have actually told the pax to do what they did?
It's one pragmatic solution to the problem ( as long as the loose stuff ends up being restrained by the strut/bar at the front of the underseat void - your void!!).
As for it being "your" space I once saw (on Easy) several passengers have their own items handed back to them from an overhead locker and being told to put them in front of their own feet in order to create space for a single passenger's oversized item which would only go in the overhead.........guess the cabin crew had been dropped in it by the gate staff and loading the large item in the hold would have meant a delay.