Low pri is inconsequential. It's 100 metal. For most factories that's not even 2 raiders.
The cost to build storage is not the perceived problem.
The perceived problem is that higher storage capacity means more time having idle metal, and players complain about idle metal.
The retort is that pointing the finger at storage specifically is exactly the same as pointing the finger at idle units, making new units that are already countered by the enemy, making expensive defenses in the back of the base where the enemy is not... In other words, there are plenty of ways people can spend metal other than storage that is just as unhelpful.