Exactly so. As deep into the game as I4 the devs were still designing around archetypes as if they had far more fixed roles on teams than the players felt they should, and were still thinking in terms that trinity-style gameplay could still be salvaged. The "everyone should be able to solo" informal design rule didn't really become a genuine design rule as far as I know until CoV. Look at CoV archetypes, and notice the shift in design. CoV is obviously "post-trinity."
But if the devs actually had a strong grasp of how to make the numbers do what they wanted, we would have never gotten the City of Heroes we ended up with. To put it in direct terms, the devs did not have enough control over the numbers to fine-tune archetype power between "lame" and "overpowered" so most things ended up overpowered relative to the design. At launch, powerset combination meant more to your overall power than archetype. Invuln/SS tanker? Godzilla. MA/DA scrapper? Doormat. Dark defender? Awesome. Electric/Fire Blaster? Roadkill. Emp/Electric defender? Useless except as a sidekick. By the time they figured out how to fix it, it was too late to fix: City of Heroes became the best MMO in the history of MMOs to solo in, because the devs handed out powerlevels most admins don't have in other MMOs. They couldn't bring down the overpowered stuff to "correct" levels except in the extreme cases, so they were forced to buff the players upward to balance archetypes out. Players always complained about nerfs, but the history of City of Heroes is an 80/20 buff to nerf empower-o-rama.
Incidentally, the last vestige of trinity-thinking lasted surprisingly long. It was the legacy of Blasters having no alternate design ethic once trinity was no longer the law of the land. In my opinion, it wasn't until Arbiter Hawk started to look at Blasters - literally in I23 - that I think Blasters escaped trinity-thinking; the last archetype to do so. Also, just a bit too late because I24 would have been the first issue to contain the start of non-trinity Blasters (one of my regrets was that I think in AH I finally had a Blaster ally on the dev team that thought about them somewhat like I did, and was willing to experiment with them outside the glass-cannon trinity model, and I'll never know where we could have eventually gotten Blasters to).
In any case, none of this was even remotely by design. It was 100% accident.
I recall another game that ended up a crown jewel by accident. Deus Ex was originally meant to be a pure stealth game, but the developers accidentally went a little overboard in providing to many weapons for the player....
...then they added a lot of combat augmentations....while forgetting it was a stealth game they were designing.
The best part of it was, thats what made deus ex so great. In doing those two things, and of course adding the skills system in addition to the augmentation system, they made a beautiful role playing game/first person shooter hybrid that rpg players and fps players could both seriously enjoy. Heck its a game I seriously say everyone should play. I know some people avoid fps's like the plague, but those people could probably get through deus ex if they'd just think their way through it. It's a game where by accident actually does allow you to get through it with little to no shots fired from a gun. You don't have to be the crack shot to win even on realistic.
And unlike system shock 2 before it, you didn't need X or Y skill to beat the game. In fact this video series shows beating the game through absolute max creative use of crates, no skills, augmentations or inventory items allowed;
https://www.youtube.com/watch?v=kAVRpvgYVMg&list=PL67B3634D970D5603 .
Just as deus ex was one of those games that accidentally entered the odd side of the game industry as a true freeform game, city of heroes accidentally exposed the mmorpg to non trinity play in the truer form, unintentionally, but would it want to go back giving up all of that freedom?