A few years ago there was a recall of OnePlus cables that were melting and catching fire, I had 2 of them and both melted.
But yes 240W/48V/5A is insane for a spec that was originally designed for 0.5W/5V/100mA. I suspect this is the limit for USB charging as anything over 48V is considered a shock hazard by UL and 5A is already at the very top of the 3-5A limit of 20AWG for fire safety.
The advantage of USB-C is the power negotiation, so getting the higher rating only on circuits that actually support it should de doable and relatively safe.
The OnePlus cable melting give me the same impression as when hair power cables melt: it's a solved problem, the onus is on the maker.
Users needing the 240W have a whole chain of specialized devices, so buying a premium cable is also not much of an issue.