Whether or not tash is a stupid spell can be argued, but making it a proc on a bracer at 100% cast makes no bracer an upgrade to that bracer without the exact same proc. It saves crucial start fight time, a spell gem (which as you said is huge), and basically makes every spell you cast less resistable, and does the same for the raid. I think if we have learned anything recently it is that things that are "utterly perfunctory" can not be fixed with items without running into thaz syndrome. And in this case it is thaz syndrome with a twist - this bracer was randomly made 100% drop rate off of a singular mob, meaning every fashionable enchanter alt in town was able to get one before it was taken back to a reasonable, less assured, drop.
Who would give up that proc for 100 more mana? 200? 300? 500? 1000? What bracer without that proc would be worth equipping over it? Even if a bracer had 1500 more hp and mana, you would still equip that bracer at the beginning of every fight if the fight had any sort of time/mez pressure.
If we want to talk about enchanter spell slots it is an entirely different conversation. If tash is really that much a waste of space, then perhaps it should be changed or removed completely. I am all for talking about spell slots if they really are the misery of enchanters everywhere - but an item is simply not the place to correct that problem given that items need to be upgradable. I am also sure that a lot of classes are enviable that there are so many extremely powerful enchanter spells that the biggest constraint on enchanter power is them not being able to fit them all on the bar.
Making the item a clicky would help make upgrades realistic - and is a change I might be open to, but would still make the bracer absolutely irreplaceable and nearly mandatory. A lesser evil to be sure, but still not something we want very often.
I thought the balance of "This is a really magic resist intensive fight - I should use real tash" versus "I need to save time, a lesser tash is worth the time/spells saves" struck a good balance.