That lethal strike will make any melee attacker (with multiattacks) outburst any spellcaster. Am I wrong?
You're absolutely correct. I just did the math. Considering 10th level and single target damage, the Immortal Mystic can do a whopping average of 352 damage over 8 turns. This is considering the Order of the Immortal Cutting Resonance feature, a greatsword, 20 Strength and no multiclassing. For comparison, a Draconic Sorcerer (Fire dragon ancestry) at 10th level who uses
Fireball as their action and quickens a
Fire Bolt every turn possible (three 3rd level
Fireballs, three 4th level
Fireballs, and two 5th level
Fireballs; 10 sorcery points, 2 points to quicken
Fire Bolt 5 times, bonus action subsequently to burn a 2nd level slot to quicken another
Fire Bolt on the 7th turn) does 354.5 damage total, on average.
This is with the Sorcerer using all possible resources available to them (all relevant spell slots, all available actions, and all sorcery points), and a frakking
Fireball on every turn. In comparison, if I'm reading Mystic correctly, this is with the Mystic only using a main action every turn (Unless using Lethal Strike uses a bonus action to activate, and even still...). This isn't taking into account the +1 "magical" bonus for concentrating the Psionic Weapon discipline in the first place (that brings the total average number to 360 on a single target). This is without the aforementioned Extra Attack class feature.
EDIT: Updated to account for the Draconic Sorcerer Elemental Affinity feature.