Nothing.
I think I wouldn't have made a big deal out of it for a short 1 hr flight, but agree that the considerate thing to do if your underseat storage takes up part of other folks room, is to ask if it's OK, and if not, move stuff so it doesn't.

If I get into my seat before the people next to me, and the folks in front, I make it a habit to stretch my feet out into my space, so that any latter attempt to intrude will be discouraged visually, and in some cases where a bulkhead seat passenger in front attempted to blindly stuff something under their seat(and onto my feet) made it impossible for them to do so.

I don't always have my feet out in front, but by doing so during boarding, it helps preserve the room needed to do so during flight.