death spells also have several counters that aren't very difficult to acquire of a decent level to block all death magic attempts
Most instant death spells are high level, to use one of Mayvinds characters (since other mages i have played against are still here and I'm not going to broadcast their DC's) had a DC of a 7th level spell at 28.
Very few characters can bounce a DC 28 of their saves, and before you start going on about KD, you may want to remind yourselves that Acid Sheath will kill most characters in 3-4 hits depending on level and by the time you've tossed out a dispel of one type or another, they have hit you with a one-shot kill spell or a timestop.
Also spells like Greater missile storm do not allow a save and do 120 points of damage on average, 240 if maximised.
You are mistaken in multiple points here.
For one. Most paladins, barbarians, clerics, and such at level 15 would have more than a 50% chance to save a DC 28 death spell. There are also death ward potions existing, which would need to be dispelled first. As an example, my unbuffed paladin (level 16) has 55% chance to save a DC 28 death spell. Buffed, that chance is around 75%. If he's prepared, the mage has time for 1 or 2 spells. (I suggest empowered damage spells)
For two, at caster level 20, acid sheath's average damage is 43,5 per hit. Average level 20 cleric has unbuffed HP of 200, needing five hits. Buffed, it's six hits. Barbarian would need a lot more hits. Clever rogues have their acid robes at hand,
and on top of that Lesser Spell Breach (easily available, castable by a level 2 bard, wizard, sorcerer or a level 10 rogue with UMD - from a scroll, which are cheap and readily available in loot) is likely to remove Acid Sheath. You can also IKD, heal, do it again.
For three, average damage of G. Isaacs on PoTM at caster level 20 is 70, not 120. Maximized the average damage is 100, not 240. empowered it is 105.
It seems to me mainly that the abundance of the +5 varnishes is an issue. I'm not sure I'm convinced it's obliterating mages' chances in PvP however. It may mean that you have to run and keep distance until you are able to dispel, have friends to melee (anyone recall the term meleers?) and act as support, throw in a summon to divert the strikes, or otherwise. I'm not perfectly sure it has to be a bad thing that spellcasters may have to do more than stand their ground, hurling spells while premonition wards off attacks.
I'm not saying that I'm certain it's not an issue either though. But have we had concrete experiences with these things? Have lower or equal level fighters proven to generally overpower mages now?
Even if made harder to get, certain groups of players will always farm enough of them to be used in most crucial PvP situations. And there are already so many in circulation. It's mostly that there's absolutely zero time to even try doing anything if a sneaking rogue, or g. sanced cleric with +5 varnish attacks you. On other hand, I'd like to imagine that something as powerful as +5 enchantment is reserved for pure casters, be that cleric, wizard or a bard.
Before +5 varnishes a mage could hardly just stand still with premonition if there was a high level rogue, cleric, paladin or a talented fighter about. Plenty of tools to get rid of the premonition as it was, but those tools required more risk than throwing in a varnish.
In PvP, summon doesn't divert anyone who's good in what he does. It's run over with haste potion, knockdown, die mage, die. I also know at least one example where a rogue took out a high level mage in around 3 seconds with +5 varnishes.