Play style is also a factor. Some people gravitate towards melees and others gravitate towards casters. Most people play both, but even so many have a play style they prefer.
I think it's a fool's game to try make all classes equal because most of the measures are subjective and much of the ranking depends on the content you run.
I disagree soloing should be the measure of class strength - it should be end game raids. The reason is that we've seen in the past where certain builds were rejected for raids - best example is arcane archer pre-U14. DPS was too low relative to other builds so in raids that once were a dps test like echrono you were a liability because it would take longer resulting more mana usage by healers and casters in general if people weren't pulling their weight. Nobody should be a liability in a raid.
Soloing it a choice, and one that is already well supported in the game. If someone wants to solo it's up to them to choose builds that work for soloing for their skill level (there are people that can solo on any reasonable build) and not Turbine's responsibility that every build works equally well for soloing.
Warlock Blasters are good but not great for LE raids. Blasters are clearly great for running quests with many mobs - who cares it's quests that any build can complete and nobody will be rejected from party due to their build.