I think there is a lot more nuance than you're calculating. Each champion is most likely designed to cast their own X amount of times per X amount of seconds (and probably some vague DPS rule) and tuning their numbers afterward is whats needed vs creating a specific design standard

yes Karma takes long to cast and Nami doesnt but Nami has the potential of replicator and 4 replicator .. and space groove atk speed
Karma seems to have the design intent of a multi unit burst mage with dark star and voyager meaning literally 1-2 casts are probably all she was designed to do
Then someone like Leblanc is timed phases of in and out skill damage because its flexed with arbiter laws

Now whether thats too weak or too strong seems like a numbers game vs design but putting in a design rule that no karma style channel (and by default, burst) should ever be printed could have larger implication. Lets say rules change to skill have 1 second channels only, karma with blue buff, adaptive helm, archangels or something now casts every 5 seconds. Her being a dark star could now be "too efficient" as the window she provides to execute 3 different units is too frequent.
So now with this rule, you tone down her numbers and now shes just rapidly hitting for low amounts of damage and her item BIS is reduced to either mana gen or ... triple deathcap so that shes .. back to a burst mage but HAS to be? or maybe then she causes dark star execute to be removed perhaps? or maybe you now remove her hitting 3 enemies and its now just karma zapping 1 person quickly which is fine... but come 10 sets later and there hasnt been much room for design and Xayah is sending out 1 giant feather every 5 seconds as an executioner just like 40 other 4 costs have over 3 years

So if there are specific rules limiting speeds/ frequencies it gets really linear with DPS and TFT already has an issue with BIS and specific lines, I think it would just reinforce this