Nobody (in the general sense) actually cares about decentralization; it serves no utility in and of itself. People only care about the ability to easily transact regardless of borders.
> Me: Cash has the same theft-irreversibility issue
I'd argue this isn't entirely correct. A cash holder can insure against theft of cash kept on premises, and in transit to a deposit facility; professional services can be hired to transport cash. As far as I'm aware these protections don't exist for cryptocurrency
> The comment only works if you assume Bitcoin is only used for speculation
At this stage it appears to me that is it's only (major?) legitimate use. Other uses probably include money laundering, and the exchange of value between parties involved in illegal activities, or for the purposes of evading regulations.