As a part-time EnchanterGuy I would definitely rather have the 100% worse proc than a 75% better proc but I still don't get what is wrong with a 100% final tash proc. It's a 1 second cast unresistable spell that costs 40 mana. How is having that as a proc overpowered in any way? Other than that there's not much that I can add on to what Tevinter and Alarcon have already said they pretty much nailed it.
The problem (in the dev's eyes) is that by having the best available spell as a guaranteed proc makes the item not only a free spell gem, but basically only upgradable by an item with the same proc. Making it a worse version of the proc allows for different itemization in future content. Realistically though, MR is already debuffed by bards/shamans to a point where having the extra 19 MR debuffed from Tashanian has little to no effect(Im a bit surprised people care enough to want to change the item tbh). That doesnt even include the fact that a majority of magic based spells (damage or otherwise) have HUGE -resist adjust. Misery of the Swamp, Crippling Surge, Intensify, seem to be the only spells without a huge -resist adjust for magic based spells (could argue mage relic rain as thats only -50)
Having Tashanian as the proc effectively makes it so that the spell NEVER needs to be loaded again, even in the future with 700 hp 1k mana 100% gather mana proc wrist, because you can just swap in the bracer for the best tash proc at the start of a fight, then swap it out after you proc (this is generally unfun gameplay mechanics in my opinion, swapping an item in until it procs, then swapping it out). Having it proc a lesser version adds a level of depth and forces you (the player) to make a choice, Mem tash and potentially demem for a spell later, or keep a spell loaded and not have to mem a spell but lose out on -20MR. Tashanian proc removes that choice.
Honestly Tashanian is a poorly designed spell, particularly in SoD where aggro on a mobs engage is much less of an issue, or spells having large -resist adjust, or mobs just having low resist in general. Its bad design because 1) You always want it (unless the mob is completely immune) 2) It has 0 impact on mana management 3) It has little to no risk and 4) It last well beyond the duration of most encounters. It doesnt have a real impact on mezzing because anything with enough MR to that "requires" tash to land mez reliably will just be Relic Mezzed anyway. Putting a poorly designed spell as a guaranteed proc is just a workaround of the issue imo (a clever workaround IMO, but opens up a whole new set of issues). Working tash into the class to just be an innate/always active thing (Preferably not a tome due to how finicky they are in terms of drop rate, and putting class defining/balancing mechanics onto a part of character progression that is so heavily reliant on RNG) could help solve the issue, but thats a different discussion for a different day.
This bracer will be hard to replace unless the item intended to replace it has a proc tailored toward enchanters given the unique design of the class. I could go into much more detail about enchanter wrist procs but basically the general idea is that very few of them have any real effect for enchanters that wouldnt be out of line for them (AKA gheal procs). Tashania is a very good (currently the best) proc on any enchanter bracer in the game. Making it better makes it even harder than it already is to upgrade, as whatever future bracer will basically need either a focus/worn effect specific to that slot, or a proc geared only toward enchanters (you could make a self only proc buff that boost spell damage output, but doing this is very very tricky to balance between an item that is too strong or undermines earlier clickies or spells like cunning and a proc that is completely useless).
Sorry this post is kind of poorly organized and doesnt have a real argument/purpose, its mostly just a collection of a few of my thoughts on the situation