The ICs don't have a high failure rate when the outputs don't fail. I worked on a LOT of these amps and saw very few failures unless the outputs failed. I've never seen any manufacturer use a heatsink on this IC.
The mass/weight of the heatsink could cause the IC to pull out of the heatsink when subjected to vibration (and gravity). If this doesn't happen, the heatsink won't do any harm.
The mass/weight of the heatsink could cause the IC to pull out of the heatsink when subjected to vibration (and gravity). If this doesn't happen, the heatsink won't do any harm.