People with 9-10 shield skill, ahem leoking, kinngrimm, etc... These people's shields are already very strong, many rounds end with 4-5 people hacking one of these shields until it somehow breaks after 30 seconds of constant hammering.
I am a shielder, and I think that shields should not be too strong, just a support against ranged and non-shieldbreakers that if you play properly will last the round, if you go against a shieldbreaker/maul, you deserve your shield to break (a lot of times i just sheathe it and manual block anyway...)
So if you want more durability in your shield, get a better one, or more agi and shield skill, its fine as it is.
Yes, make shields indestructible, and at the same time make 2hs extremely slow, make half of an archers screen to be black, make crossbowmen start dancing and drop their bolts when i stare at them, make poles lose their polestagger and remove their stab, but buff throwing because i want to use a throwing lance with my shield and 1hit everyone...