I don't know how you get 70%. Again (respectfully), I ask: how sub-optimized are you talking about? Because the example character I posited can hit AC 43 almost half the time (40% chance for what I consider a REALLY non-optimal character). Throw in other peoples class buffs (especially at high level) and there is no reason to expect you can't hit the higher 48 or 49 AC half the time. And, again, that example character is not counting any weapon prof. bonus.
70% seems pretty ridiculously high miss chance, and I'm wondering if these characters you're talking about are worse than "sub-optimal".
Better check your math. An attack stat of 24, a +15 level bonus, and a +4 weapon comes out to +26, not +30.
Now, let's make the weapon a bit more plausible. +4 is really quite low - you should have at least a +5 by that level. Since you're not optimized, though, you maybe went with a battle-axe instead of a nice accurate sword. So, a base attack stat of 16 boosted to 24 (+7), level bonus (+15), and a +5 battle-axe (+7), comes out to +29 with proficiency bonus included.
Against the tarrasque, that means you have to roll 14 or higher, giving you a 65% miss chance. Then consider that the tarrasque is a brute, and its AC is a bit low even for that. A "normal" solo of that level would have an AC of 46; 80% miss chance. An ancient red dragon, being a soldier, has an AC of 48; 90% miss chance, meaning you have to roll a 19 or better!
Before Expertise was released, even your optimized Demigod-destiny buddy, with a 30 in his attack stat and wielding a +6 bastard sword, missed the dragon 65% of the time.
Now let's compare that to the same character at 3rd level, fighting a young white dragon with an AC of 18. Attack stat of 16 (+3), level bonus (+1), and a normal battle-axe (+2), comes out to a total of +6, meaning you hit on a roll of 12 or better - 55% miss chance. And the white dragon's AC is a bit higher than normal for a solo brute.
A solo soldier at 3rd level would have an AC of 21 and you'd hit it on a 15 or better. Not great, but a lot better than having to roll a 19!
Bottom line, your attack accuracy relative to monsters of your level drops by about 4 points relative to monster defenses as you advance from Heroic into Epic. So, yes, without Expertise the math is off.