I'm not seeing how full caster ruins the bard archetype.
The bard in 3e was weak. They had the problem of sorcerers, but worse. Slow spell progression, limited spell slots which played havoc on balancing spell levels. (More on that in a moment) And they got little to trade in for; a few buff-songs and counter-songs, light armor, more skill points, and a few extra weapon proficiencies. (And a rogue's HD/Bab, so they were miserable combatants that lacked a rogue's SA spike damage). They needed some work.
The biggest advantage moving a bard up to full caster does is allow their spells to scale properly. For example, a bard in 3e got summon monster spells (which fits thematically). However, they lagged behind in spell slot acquisition so badly that most of the time, when you got a new SM spell, it was woefully outdated (Summon monster IV came at 10th level, the same point clerics and wizards got V. At 16th level, they gained access to the same monsters a cleric had at 11th). This was similarly true of illusions, charms, and healing spells; they didn't even do their magical shtick as well as the core casters. Moving them to full caster puts them on par with a cleric now. They will get access to healing at the same rate, can use charms, illusions and summons like a full caster, allowing someone to probably play one as a replacement for a cleric or druid (yay!) rather than a second-rate caster.
In fact, comparing them to a CoD might be an apt metaphor: full casting to keep their magical abilities scaling, a bit of weapon/armor use (since their magic isn't going to be combat-based), and some funky powers (inspiration, skills) to equal domains/wild shape.
I mean really, who EVER complained the bard was "too powerful"? Full casting is probably a simple fix that would do a lot to making them on par with the divine duo (which seems to be the new role they are going to play; a nod to the 4e Bard/leader).