Prism
Explorer
The difficulties faced by a bard when using a spell focus as a full spell caster are different than other full spell casters in the game. The difficulties emerge both in game mechanics and in the rationalization that is required to make sense of how the rules can be made to work.
The fact that one can invent situations that can be made to fit the rules is irrelevant: other full casters don't face these arbitrary constraints, and the rules do not seem aware of their own inconsistencies.
Its not really the bard that's the problem, its just that the other classes either don't use a shield or often use two weapon fighting. It would be a problem for eldritch knight or arcane trickster too but they don't get to use a focus anyway. The bard without a shield or two weapons is just fine.