People have tried a number of things to balance them lol, but if you adjust their stats, they become crap late game...adjust the skills and they do next to no damage because of the way they scale from their mana...lower the mana buffs and you get the same issue.
Without putting the effort in like you said, and we all know that isn't going to happen, it's not going to be possible to balance the class properly with the skill set they have without some adjustment in some form or another...and people hate change.
Also don't forget, even some simple changes Zircon did like not having that face down motion when you cast Defiance as a Warrior and it made you stand in place for a second or 2...Jamie removed that bullshit and you could cast it like you were casting a skill...small things like that made the version feel smoother but could also have impacts on the responsiveness to classes.
I never saw/played Sin prior to Zircon, but I would imagine it's backstabbing teleport thing had a similar delayed response as Defiance did...without it would make them really tough to counter...but Zircon just by it's smoothness, likely made them even more powerful if you think about it as you didn't need to contend with the inevitable frame delays.
Zircon has improved Mir3 a great deal. I'm all for being oldskool, but without it...Mir3 would be dead by now. It would be very easy for someone to adjust the files as they are and made them more familiar...but people can't be bothered lol. Only person who attempted was Zed, and then he locked the source
Jamie was more focused on responsiveness and actually being able to play the game how it was intended without spending years on the aesthetic so called "feel" of the old files, not the buggy/laggy state it was known for.