Hmm, I don't think it's actually that different between 3.5e and 5e once you're high level. AC is universally lower in 5e compared to 3.5e. Recall also, in 3.5e your subsequent attacks each take a -5 penalty (i.e. a lvl 20 Fighter BAB is 20/15/10/5)
A 3.5e lvl 20 fighter with +(35/30/25/20) AB vs. 39 AC = hit chance of 85% / 60%/ 35% / 10%
A 5e lvl 20 fighter with +14 AB (x4) attack vs 22 AC (Ancient Red Dragon) = 65% / 65% / 65% / 65%
Epic level is not lv 20. Is
above lv 20. 3.5e and 2e din't followed the "tiers" that 5e sadly brought from 4e. An lv 1 mage is a guy who casted the first spell today, a lv 20 is the guy who mastered the arcane arts and above lv 20, he broke all human limitations.
Also, you are ignoring that attributes, feats and gear has a HUGE IMPACT on Attack Bonus.
A epic level warrior would probably have a +5 weapon. Magical items and weapon focus, greater weapon focus, epic weapon focus and weapon specialization.
The same applies to spell resistance on 3.5e. Most casters would have spell penetration and greater spell penetration and some enemies on kingmaker like spawn of rovagug are hard to hit even at lv 20 with both feats. He has 42 SR if I remember correctly.
He is a hard enemy due regen, resistances, immunities, summons, powers, damage dealing, etc; not only due a hp bloat...
