You CAN force a horse to become round - I could name a couple of devices that will achieve exactly that ( and a running martingale is not one of them )
The main argument is that a horse , if properly trained , will become round all by itself. If a horse is forced into an outline then often this will create an imbalance due to the horses muscle structure not being ready for it.
You also quoted earlier that your horse wore a SM for years , and that you took it off as you would rather have your horses head up rather than forcing it down, did you do no training in those years to correct your horses head carriage?
. My own horse wore a RM for about six weeks and during that time I trained her properly , and then took the RM off as it was no longer needed