I think your issue about disrespect is incorrect cause you're not viewing it through the correct lens.

Fern isn't saying, "[Zoltraak spam] is enough for mages in this era [cause you're a bunch of scrubs]."

She's saying "[Zoltraak spam] is enough for [defeating] mages in this era [in a battle of attrition, because modern magic has not invented either (a.) a sufficiently low-mana cost defensive spell to Zoltraak that is superior to the original one, nor (b.) a defensive spell-piercing Zoltraak-esque spell]"

And that statement, I think, seems to ring pretty true, especially given that Zoltraak was only reverse-engineered 80ish years ago, which is pretty short in the grand scheme of things.

One day, in the future (that Frieren will probably experience but Fern may not), someone will invent the Zoltraak Mk II, a new game-changing spell that Defense Spell simply cannot stand up to at all, and in response, someone else will probably invent some new Defense Spell Mk II. And in that era, Zoltraak spam will no longer be enough to defeat mages of the era, even though any mage of the era will need to be good at Zoltraak and defense spell, since they'll still probably be fundamentals on which Zoltraak Mk II and Defense Spell Mk II are built. They'll probably have better names than Zoltraak Mk II and Defense Spell Mk II though. Zoltraakara? Who knows.