I actually do think armoured horses are, well, too good.
However it's fairer to say "let's remove them" then do something like this, and return people their loompoints. Upkeep clearly doesn't work, and I agree. However, doing this is tantamount to deleting all builds for armoured horses except the obviously undesirable ones (10 riding, 10 shield skill LLJK style +3 plated charger for trampling people with shield up and wearing a wedding dress, if you remember that particular character). If you want to say "delete armoured horses" then say it openly.
And no, the listed example you gave is 6 slots. Read your own proposal (2 slots charger, 2 slots kuyak, 2 slots heavy lance).