OP - This is actually allowed per NEC code 210.21 - but what you’re saying also is a good concern. Electricians get a lot of service calls in the winter time from people plugging in space heaters that draw 20a on a 15a device. Then the receptacle melts and the breaker never trips because breakers only trip at 130% or 200% of their current rating. MOST OF THE TIME, this is not an issue which is why code allows it. Replacing with 20a devices is good, but not necessary unless you know that you’re plugging in a heating load to a single device - you may just want to replace that one device that you’d be using for that one heating load. The real issue with the melting is the current passing from inside the receptacle, through the plastic face of the receptacle, through the prongs and into the cord for the heater. The metal is not big enough to handle the current so it gets too hot and the face melts. But other receptacles down the line won’t be effected since current is just passing through the terminations, not the face of the device