I won't dispute your theory. It was exactly the same logic that led me to start to give such a power to my solos, more than a year ago. I just found it unbalanced my solos too much vis-a-vis my players. Maybe they (my players) aren't as optimized or focused on stun-lock as yours? Not sure why we arrive at different results.
There are a couple of things I should note in that I use a mixture of different mechanics for solos, but I do have a common rule:
1) Most solos are specifically resistant against the daze, dominate, stun and unconscious conditions. These are the conditions that make a solo fight immensely trivial, so they're the ones I give special consideration towards. Solos are not immune to
any other condition and don't make saves against other "until the end of X turn" conditions. It's important to remember that this is where the true flaw in 4E solos is: until end of next turn conditions absolutely wreck them. It's not your regular save ends powers that are screwing them entirely. Noting that this is an end of turn save, not immediately and I don't always give them an immediate saving throw. It depends on the monster and the goal it has in mind (see below).
All solos above paragon tier in my games can save those conditions at the end of their turn, regardless if they are save ends effects or not. Some will simply automatically end these effects.
2) Different mechanisms
A) Some solos will save the instant the condition is applied. Common examples are dragons and other non-multiple turn solos.
B) Multiple turn solos lose their next activation. Depending on the solo multiple daze and stun conditions may or may not stack (2 activations, usually they never stack, 3 or more activations they often will stack).
C) Multiple standard attack solos lose an attack off their standard action. Jerusha for example loses 1 attack from elemental barrage per stun, dominate and daze condition imposed on her, but otherwise is unaffected. Given she attacks up to five times with it while bloodied, she can put out a considerable amount of damage with that power. This is based off the "many headed" power from the Hydra.
D) Dual brained monsters, like an ettin, molydeus and similar (this is in reference to elites actually) automatically end any daze or stun effect imposed on them at the end of their turn. Solo dual brained creatures with multiple turns default to B.
E) Some solos will impose the condition on allies with them in the fight, meaning the solo cannot be locked down unless its allies are killed first.
F) In very rare cases, some solos are simply outright immune to stun or treat these conditions differently. An example in one of my upcoming campaigns is a creature that treats the daze condition as losing its move action and the stun condition reduces it to a single standard action and without an immediate action.
The overall effect is that I try to really mix up the solo mechanics a lot, either with instant saves, but others will be affected by conditions differently and such forth. The goal isn't to make the PCs feel their powers are useless all of the time, but instead to ensure that daze, dominate and stun cease being epic level equivalents of 3.X "Save or die" powers (or in some cases, it doesn't matter either way like destructive salutation). A solo fight where it actually acts and does things is far better than the following:
Hit (or miss) with stun power until end of next turn and AP -> Dump powers, next turn, AP -> Dump powers, next turn, AP -> Dump powers -> next turn AP -> Dump powers, next turn, monster does utterly nothing, next turn AP -> dump powers -> back to character that stunned the solo round one.
If the above was boring just to read
imagine playing that. With careful use of powers you will never ever ever be damaged for any reason except by an aura or no-action trigger.
The one exception I have though to what I wrote above are deities, demon lords and other "level 30+" creatures. They are basically your "end game" creatures and deserve to have a power level in excess of other solos or creatures. Unlike my normal solo rules, they can make a save against every condition imposed upon them (except for marks, which I view as a special exception to this). They are suitably powerful being the level they are and given that 30th level PCs are utterly
brutal on many levels, this works out pretty good. It's only a mechanic I'll use once or twice a campaign as well, making the final ultimate encounter of the game especially high stakes against PCs at the very height of their power.
Edit: On resisting elemental damage as I like to mix up mechanics, one of my upcoming epic solos Lilith has an aura "Strength to Weakness" (yes I love my long power names sometimes). It's an aura 5, it turns all resistances in the aura into vulnerabilities of the same time and amount. That will give them something to think about
I don't really mind resistance to elemental damage though in general. I think the key thing to do in any encounter is to mix up your monsters and damage types. If you have a mono-damage group, don't be afraid to give them ways around resistances, impose vulnerabilities or similar.