I think ultimately that many things are possible, but if they didn't happen there was a good reason for it.
I mean, there have been some pretty smart people around since the dawn of time. If there was an advantage they would have seen it and adampted it.
Sword and Bucklermen in the 1500's had a 'bullet proof' shield, so the concept of bullet proof sheids is nothing new, but the musket still won out at the end of the day. If the bullet proof shield was so effective sword and bucklermen (or their evolution) would have been seen in vastly greater numbers.
IDK, there have been a lot of instances where ideas which seem obvious in hindsight didn't occur to people for long periods of time. E.g., gunpowder has been kicking around since the 10th century, but nobody really utilised its full potential until Early Modern Europe.
As for the sword and buckler men, I thought they were discontinued because they were only really useful for breaking the deadlock of a push of pike, which didn't really happen all that often, not because of any problems with bullet-proof shields.
(Incidentally, sometimes military equipment is discontinued before being re-introduced later. E.g., breastplates were out by around 1700, and were off-and-on-again over the next century until staging a mini renaissance thanks to Napoleon's cuirassiers. So I think that in some cases, changes in equipment is due to fashion rather than viability.)