Please read. I never denied it needed to be long so you mentioned a moot point. Also the DoT being long is necessary because of how it works is fine, yes, but it is also to it's benefit as it's one of the stronger Dots and very easy to manage compared to Dots usually lasting 20. And I said 50 potency on the DoT with KAITEN. And specifically said not including Jinpu. What this tells me is that you probably don't use Kaiten on Higabana and so you lose out on DPS already. So much for being max level to understand the mechanics.
You mention the Dot needing to tick longer like people seriously would use it on something that doesn't last. Overlapping Sen is near impossible if you are actually aware of your options like Hagakure which is almost always a gain if you have it. By the way, that is just being aware of your situation, that doesn't necessarily mean you're good. If you don't and need to apply buffs, yes, you have to deal, which would probably still be rewarded with a 720 potency attack. Tell that to the other classes that don't have much for those situations.
Also lemme answer your scenario with a scenario: A10S with it's plenty of mechanics forcing you to move, along with the boss moving beyond tank's control, and a bound mechanic that will keep you from moving or even turning. You won't have True North for all of that. From what you said, SAM will only lose 60, meanwhile you have DRG still losing more than 100 should you proc FnC followed by WT out of Chaos Thrust. God forbid they have a boss with all of that plus A11S with forcing people to hit from the front to break a shield. In that situation with no True North, SAM still doesn't miss out on much.
I love SAM as a class and for the aesthetic since I wanted a sword wielding DPS for so long, but let's face it, it is easy as sin and rewards you way more than any other class at the moment. They had more thought into them and it shows.