I understand your assumptions, but no.
Both spellcaster players chose their choices in order to get Fireball and more generally, kick ass personally. Neither player is much interested in sacrificing their own glory to let others shine.
In which case, they picked their classes poorly. Neither the Cleric nor the Bard really kick ass on their own. The Lord Bard shines when buffing her allies and debuffing monsters so the monsters can't hurt said allies; her best control spells aren't meant to hurt. Calm Emotion and Enthrall are actually great combat preventers for more social games, Blindness/deafness as well as Hold-Person class magic and Hypnotic Pattern shuts down monsters with a failed save - at that point, the monster is all but dead. Command and Compulsion can force monsters to walk away from your melee warriors, prompting attacks of opertunity.
The Light Cleric is admittedly kinda sorcerer-like, but trades in spell recovery for Warding Flares, which functions a lot like the the Bard's Cutting Words feature, and Channel Divinity, a rather lackluster spell, gets damage bump later, and lacks the metamagic to boost attack spells upwards. Outside the Light Domain's spell list, there's not a lot of direct damage abilities, though Command/Hold Person class magic and the Spirit Weapon* class of spells are very effective for clerics, which should not be ignored.
* Spiritual Weapon, Spirit Guardian, Guardian of Faith, and Blade Barrier all count, and all highly effect coupled with Planar Ally, Conjure Celestial and Gate at high levels. Guardian of Faith, Spiritual Weapon, Planar Ally, and Planar Binding (Conjure Celestial/Gate) dont' use Concentration even!
Honestly, if its not deliberate, then its very possible that the players just don't know how to play the class. Both the Light and Lore can be very effective with the right style of play; it doesn't seem like they were using that style.
Sure Bardic Inspiration was nice, but hardly a game changer.
Bardic Inspiration is actually a game changer for GWM if they aren't consistantly hitting on their own. Cutting Words is very effective when trying to protect yourself against someone attacking a caster who doesn't want to drop a Concentrations spell as well. Its really good to add onto skill specialists (seems you didn't have anyone) as well, letting you be a bit more brash than not. They're quite effective if the playstyle allows it.
So the situation isn't like your theory.
Both martials outshone the casters mostly by themselves. They could not have done so without access to bonus attacks nearly every round.
The -5/+10 mechanism is just icing on this cake, the final straw that propels the barb into a definitely higher division of combat usefulness.
Well, its not quite a theory, more like an attempt to play devil's advocate. You said that the people were used to 3e optimization, so I assumed that they would have figured out that the cleric and bard weren't very strong controllers through damage dealing on their own. It appears that this is not the case. Perhaps they were used to caster domination of 3e, and carried that false assumption into 5e. There have been a lot of effort spared into making sure that the classes are balanced and the casters don't outdo the melee warriors in terms of pure damage. Now, pure damage is actually what melee warriors are specifically designed to excell at, while other classes have other roles they fill.
It just sounds like the Light and Lore weren't played to their full capacity. Now, I'm not there, but that's what it is increasingly sounding like here.