That’s really only true at lower levels, I think the complaints are mostly WRT level 100 content.
If you have a 0GA item with the right base affixes which rolls well on the Weapon temper, it’s got something like a 30% multiplicative bonus. Even a 3GA with the right affixes probably doesn’t beat that if it whiffs on tempering, the Weapon temper specifically.
Not sure from memory what the other options are if any. Is it possible to choose a different manual which isn’t hit/miss? TBH I don’t think I have looked as I always focused on getting the best affix. If that does at least exist, so you have an alternative which will almost certainly give you something decent albeit reasonably inferior to the Weapon affixes, then the Weapon affixes could be seen as aspirational high-risk affixes.
So when you get your first 3GA weapon, you can play it safe to make sure it’s a decent upgrade albeit imperfect. Maybe you get that with Tempers still remaining. Then if you find another, you can try the high-risk manual on that.
Lots of way they could go which would improve things. Complaining about bricked items would be less reasonable if there were safer yet sub-optimal manual options.
To be fair, the devs did say they want to keep expanding on the Tempering manuals so maybe this is what they envisage but just didn’t have time to implement/test yet. Or maybe it’s already that way and we are overlooking it.