(1) Paragons became stronger with more shouts, Guardians trade-off dps etc with team buffs. The fluffer for example is described as fairly useless outside support.
(2) Having more than 1 guardian is useful not because they are synergistic, but because they can use skills in sequence, i.e. I put up a wall of deflection, when it goes out another puts it up. This means more guardians need to cooperate more strongly not to step on each other's toes. Paragons at least had some bonus buffs from other paragon's shouts.
(3) I think a lot of the functions done by guardians can be done by other professions, but people have not tapped into that yet. Players who pick guardians often do so because they like support. But them mesmers can support via a number of skills and traits, and I doubt we've seen many players trying that out.
I think the standard definition of trinity requires a dedicated tank and a dedicated healer.
When people think trinity, they certainly don't think about tanking with clones, everyone contributing to team health by dodging and self-healing, or the inability to make a dedicated healer. How can you LF monk if there are no monks or reasonable substitutes? Even the "dedicated healer" you are thinking of, the guardian, is a farfetch from what dedicated healers are or do. The guardian is generally tough, frontline, and protting. Frankly, it's more of a tank & healer rolled into one than it is either.
I don't think you can cry "trinity" and pass as a serious game critic. You'll have to pick either, and go with it.