I wholly disagree. I went through some testing with it and Eviolite Bisharp does pretty damn well. If Bisharp's base 70 speed is a "useless" speed tier, then Kingambit's base 50 is absolutely abysmal.
Sure, Bisharp doesn't hit as hard as Kingambit, even after a Swords Dance, but it still has similar profiling to it.
It working personally for you doesn't mean it works on a broader scale. Plenty of people have made bad Pokemon work for them. That doesn't represent the Pokemon generally being good, but the player using them. When I say Bisharp's speed tier is useless, I mean it doesn't make a difference over Kingambit's. The extra speed doesn't help it in any matchups that Kingambit doesn't already win.
It doesn't just not hit as hard, it hits so much weaker that it's a wet noodle by comparison.
252+ Atk Bisharp Iron Head vs. 252 HP / 156+ Def Garganacl: 180-212 (44.5 - 52.4%) -- guaranteed 3HKO after Leftovers recovery
252+ Atk Kingambit Kowtow Cleave vs. 248 HP / 0 Def Moltres: 171-202 (44.6 - 52.7%) -- 23.4% chance to 2HKO
252+ Atk Bisharp Night Slash vs. 248 HP / 0 Def Moltres: 135-159 (35.2 - 41.5%) -- guaranteed 3HKO
Like there's no comparison especially when Gambit gets stronger over a game and can overcome walls while Bisharp remains walled. It only has a shared typing with Gambit, but past that has significantly worse offensive presence and defensive presence.
I do want to note that whether or not it pans out, this isn't strictly a rule. Different environments can make Pokemon more viable in a tier above their usage wherein they're bad there, and if the concern is "we are desperate for Ghost-Spam checks without Kingambit" then I think people would find ways to play Bisharp to that end.
This typically applies to defensive Pokemon though. It's much harder to rationalize using an offensive Pokemon that is bad in a lower tier, in a higher tier unless it has a unique offensive profile suited for the higher tier that it ends up working in the end. With Bisharp just being significantly worse than Kingambit, it's not gonna fill that role.
And if we're so concerned for a ghost check that we're pulling out junk like Bisharp, then that's just a sign that ghost type offense (Pult cough cough) is an issue. But even there we have counterplay with Tinglu and Garganacl existing.