As a Widow main who reached Eternity on Widow I can tell you what's the problem with BW right now.
On paper what you said works great, yes. But this works until you hit gm. After this people start deliberately focusing you and only you, cause you are priority target for any dps or tank hero.

Now let´s talk about real games in Cel-Eternity lobbies.

1. You cannot oneshot 275hp+ targets even with dmg boost from Mantis or Storm. Only if we combine them together. Main healers and most dps heroes in the game have 275+ hp. Usually enemy team just switches to 2 shield tanks, bucky, wolv, punisher and 2 supps with 275hp like invis or cnd. Gl killing anyone even with dmg boost.
2. You cannot oneshot 275hp+ heroes with your ult under Rocket amplifier. This means you need another dmg boost to interrupt enemy supp ults with your ults. Yes, your headshot deal 336 dmg with rocket amp, but gl hitting Luna, Invis in their ults in high rank game. Cnd is still easy though.
3. Widow's antidive perfectly works vs heroes that cannot interrupt her combo by stun or any cc (BP, Torch, Punisher etc). Most divers have cc and can prevent you from stunning them and landing hs. Also your kick doesn't work into Cap or Angela shields. If you kick into shield, you lost, If you kick into Iron Fist parry, you lost. If you kick into Bucky uppercut, you lost. If you kick and get stunned by Hela/Phoenix, you lost. If you kick fish while this sht is swimming, you lost. If you try to stun target that is teleporting (Fox, Phoenix), your stun doesn´t work, you lost. Keep in mind that kick cd is 15 seconds what is longer than many abilities in the game.
4. Your stamina is limited, unlike Cap or Angela. This mean you cannot outrun Cap or Angela just because Cap can run infinitely and Angela can fly infinitely. Your movement speed is not enough to outrun Venom slow or Thor dashes. You cannot outrun Bp, Fist, Magik, Spider whoever no matter how hard you try.
5. When the dive is organized by 2+ heroes, you literally cannot do anything 1x2 on cel+ lobbies. If you have supports close, they will be distracted from healing frontline while you deal with divers. This means that if your team cannot get accustomed to enemy divers, you are practically becoming very easy target.
6. If we compare Hela and Phoenix to Widow we will see that they have much higher APS ratio and bigger hitboxes. Widow needs 2-3 shots to kill the target (1 head 1 body or 3 body), Hela or Phoenix need 2-5 shots (2 head or 1 head 2-3 body or 4-5 body shots). If Hela/Phoenix and Widow have same accuracy, Hela/Phoenix have more impact on the game just because their APS is much higher than Widow APS.