Why post irrelevant achievments, is this a kindergarten? Its 2018 and it makes your arguments look bad if you do, provide actual footage like my video, when rogues were unnerfed in early Bfa and still got kited to death by a class that is the worst in the meta.
First off, I never said a rogue is a bad duel class or can't be good in duels, i said assassin is. Its just the spec, unlike the other fotm spec, "disc" its not the best option for a rogue when trying to solo people, people that are prepared and not afk, that is, you can always gank people with low hp in warmode and just pick specs that cannot defend against dots and evenom spam, but you could do the same nonsense with a hunter or boomkin/feral, too and i prefere their toolkit over rogues, since legion went live and overhauled their specs.
Outlaw and Sub can be very good in duels, but assassin simply does not work without heals/dispels and the way how they deal constant dmg by applying multiple gcds on their target. You can't put them against any kiting dd, or classes/races that could remove their dots,and there are quite some that are good in that.
As far as frost mage goes, i never had a problem with rogues, certainly not assassin. Simply compare their cd durations and how dmg/cc is done with those of rogues to get a clue. Can't beat that as they are essencially rogues with high range on all abilities, could kite, and remove dots and kidneys, you need even a gap closer to blind a frost mage outside of outlaw as the range is so short and all it does is heal the mage, thanks to certainy azerite traits based on cc + a mage will instantly remove it with trinket, since kidneys are a non issue - blind/sap will never happen on a class with a pet that got full immunities to kidney to freely use trinket on blinds.
I wonder, really where you pull your 90% win numbers as assassin against frost mage duels? was it WoD? Vanilla/TBC? Certainly not BfA, even in Legion, rogues were far better in such a matchup due to vanish powder and op legendaries like the cloak with small 5 sec cd on feint and hp heals. Lets not compare apples with oranges here, thinks changed a lot in BfA and the pruning is more visible on a rogue than any other class after BfA happened. I currently play 5 classes in BfA and noticed some even got something baked into their class that is pretty powerful in duels.
The only expansion i remember when this was even out was wod, when rogue like you said really got the tools while classes were generally pruned, unlike BfA. Rogues were pruned first time in early mop, but kept more abiliteis in wod than any other class.
Death from above might help some, but if you truly won duels against frost mages as assassin in BfA just by using this they fucked up. Its still is easier to kite with superior slows and movement on shorter cds. Also both Outlaw and Sub do their burst more instantly with less gcds what is a big factor, you can't survive forever, certainly not with breaking roots by using your only combat reset button "vanish" since blind/sap will not work against a frost mage, due to pet follow, trinket on blind and combats 5 sec rule. I still use vanish often against roots, but i do so, when i am sure to finish the job. But that point will hardly be reached against a frost mage who is knowing all rogue mechanics an there aren't too many regarding Assassin's bob pve spec. Actually, if this expansion would be any good in pvp, assassin would stay out of pvp, maybe it was a big mistake to only allow poisons for assassin.
If you lose as a frost mage to those melees in duels, especially assassin rogues, rets or any kind of warriors you are truly bad and should be ashamed of yourself. At least you see the issues Rets and Warriors do have with Frost Mages.
And in terms of consisting win rates in duels why rate rogue over druids, mages and hunters? Early BfA i would have agreed, but after the heavy nerfs on burst dmg on outlaw and sub i can't, not with disc around and dd kiters, that seem to improve more and more over hotfixes and patches.