Is leaving the door or side panels off a good substitute for proper cooling?

Not really.

Removing panels may reduce temperatures in some situations because it gives heat more ways to escape, but it also changes the airflow and removes some of the physical protection provided by the enclosure.

More importantly, it can hide the actual cause of a cooling problem. If one switch has a ventilation opening buried behind a wall of cable, taking both side panels off may reduce the temperature without improving the underlying layout.

If the door, both sides and rear eventually have to come off to keep the equipment comfortable, you’ve taken a surprisingly elaborate route to inventing an open rack.

Work out why the installation is getting hot rather than using progressive cabinet disassembly as temperature control.