A monoculture is bad for resilience most of the time, which doesn't change for hardware. But would the affected fraction of the circulating coins really be that relevant?
Indeed true but what is more true is, there isn’t a chance that all the hardware wallets would ever stop working making all the coins within it lost to the network. There are always developers looking out for what possible threats that Bitcoin might face and the best way to make amends. Take the block size for an example, it was increased just when there was a need. When the hardware wallets proves to be of less use, there would be some more efficient storage to replace it.