I just really do not want to tie up a horse everytime I dismount. If you have to tie up your horse and neglect to do so what is the consequence? Will it run off or get stolen? What if you are in an open area and there is nowhere to tie a horse? Would NPC’s want you hitching livestock to their fences or property?
In RDR you tied a horse to a post. Would the Warhorse team have to set up designated posts all over the game or find a way to allow players to tie a horse to any tree, fence, ect without any problems?
I think the best solution would be to change the part of whistling you do not like. Have it so the horse does not disappear. In Shadow of the Colossus the horse Agro would keep a respectable distance from you and drink water and eat grass while you were dismounted. When you whistled he would run over to you. He also freaked out around things that made him nervous - the colossi and shadow beings for example. This made him all the more believable as a living creature.