And if you don't want to deal with that 20% chance, just Covert Cloak, bro
Getting absolutely zero "hahahas" on this is maybe the lowest point in my forum history.
In actual relevant conversation, one of the biggest differences between Quick Claw and secondary effects like Para/Freeze/Flinch (beyond those actually being mitigated by Cloak) is that those effects only apply to specific moves. As
Smashburn pointed out, Quick Claw is permanent. i.e. it applies to every move.
Fishing for a Scald Burn or Discharge Paralysis can change a game, but it's rarely the optimal play (I'll admit that Flame Body and Static are another story since they are also "always active" and unilaterally punish the other player's choice instead of rewarding yours). It's something you do to turn the tables when the matchup hasn't gone your way. Quick Claw takes the optimal play and 20% of the time makes it the super-optimal play.
Example: Your opponent just brought in Dragonite against your +2 Iron Hands after a KO. Iron Hands is at 30% health (out of non-Tera ESpeed range, well within EQ range). Clicking Ice Punch is probably the smart move to punish a DD from somebody expecting a switch or to prevent your switch-in from taking extra damage when Iron Hands *probably* isn't gonna do too much more in the match beyond serve as a sac with its low speed and hazards on the board.
In non-bizarro world, there's a simple flow chart. Opponent clicks DD, Dragonite Dies. Opponent clicks EQ, Iron Hands dies. Your opponent has agency. They ask "will they switch or will they stay in and attack" and make the correct choice based on their knowledge of the game. In this Quick Claw example, 20% of the time it simply doesn't matter what the other player clicked. Dragonite Dies. And the genius behind this monoclaw insanity is that most of the `mons being run here have the potential to OHKO the overwhelming majority of the tier after a boost.
Similar example. Let's say Iron Hands_is_ in Extreme Speed range now, but you also have a Scarf Iron Valiant in the back that already clicked Tera Ghost (because this is the world we live in now). Dragon Dance is basically not an option (lose to Iron Hands if it stays in, lose to Valiant if it's a switch), but now it's a choice between clicking Earthquake to either KOing Iron Hands or deal massive chip to Valiant but risk getting ganked by Quick Claw or clicking Extreme Speed and guaranteed killing Iron Hands but letting Valiant in for free.
Congratulations, you have trolled your opponent into seriously considering a terrible option (ESpeed) because they don't want to lose for making the correct play.
Finally, to echo one last point from people making comparisons to status: Yes, status RNG hax decide games, too, but banning Paralysis, Freeze, Flinch, etc. would be a massive change, drastically reducing the number of available moves in the game and, as a consequence, functionally upending the viability of entire types based on move availability. Banning Quick Claw would be straightforward. I'm not actually advocating for banning Quick Claw (yet), but arguing that we shouldn't do one if we won't/can't do the other is clearly disingenuous.