There's a soft cap on caster DCs of about 45 for a palemaster/archmage. Possible to get higher but it's pretty rare to see that.
To quantify it for you:
9 spell level
2 greater spell focus: necromancy
2 Prestige enhancement bonus (lich or archmage)
2 greater spell focus item
1 active wizard PL
19 intelligence (18 base +5 levels +4 tome +4 enhancements +11 items (epic, exceptional, profane) +2 ship +2 yugo +2 completionist)
3 from more Int +2 store +3 alchemical +1 drow
That's about the limit of what is possible iirc. Put all that on a rogue and let's see what we have:
3 from more int +2 store, +3 alchemical +1 drow
=54 assassinate DC.
Pure rogues obviously have the edge when it comes to pure instant kill DC. However, arcanes have other strategies available to them like crushing despair, roaring (belt buckle), demonic curse (consort bracers), symbol of death, circle of death (on a successful fortitude save), energy drain, enervation guard etc none of which the rogue has access to unless another party member is applying the affects for the rogue.