That is generally how the game panned out for the group I was in from about level 13 and onward. In the case of the group members who optimize more, it may have been earlier. In the case of one of the campaigns in which the party had two Warlords (one inspiring and one tactical,) the domination of the PCs started around level 8. There were generally some hiccups at level 10 when the PCs were not paragon yet, but were facing foes who were; likewise for the transition levels between paragon and epic. In many encounters, if the opposition was able to act before they died, it was a surprise. It was quite common for the majority (if not all) of the enemy to be dead or nearly so before they (the opposition) even had a chance to react to the PCs; this was especially true with the two warlord party I mentioned because the party had a rather massive bonus to initiative from the inspiring warlord (who was also a half-elf and took the racial feat which gave a bonus to initiative.)
Thinking back across the campaigns I've been a player in, there are two party compositions which I feel were even rougher on the opposition than the two warlord party mentioned. One was a combination of an inspiring warlord (my character at the time,) and a psion. The psion could give penalties to our enemies while I was simultaneously giving bonuses to the party. In many encounters, it was virtually impossible for the enemy to hit us. Meanwhile, it was virtually impossible for us to miss them. On top of that, even if they did manage to hit us, they were barely doing damage. Meanwhile, if we hit them, I was typically able to boost the damage output of the party.
The other was a campaign in which I --surprisingly-- did not play a warlord. (Aside from being my favorite 4E class, the rest of the group often requested I played one because I was apparently quite good at it.) I was playing a bard, and a different member of the Saturday group was playing a Warlord. Honestly, I cannot remember the names of the powers offhand, but I remember that he had several powers which allowed people to reroll attacks. At epic, he had one which somehow granted the rest of the party more attacks if he or someone he chose scored a critical hit. Meanwhile, many of my bard powers either granted extra attacks or allowed party members within a certain radius of myself to make attacks. Long story short: a lot of rolls + enhanced effects from critical hits meant we were nearly guaranteed a critical. I should note that this set up was in no way planned or intentional; our powers simply meshed very well. Add to the mix the barbarian pc who was already doing a ton of damage on his own with one attack, and it is in no way an exaggeration to say that most encounters were a slaughter. This particular campaign ended so one-sided, that I even tried to sandbag my actions during the BBEG fight (as well as most of the epic tier) so as to spare the feelings of the GM.