Higher temperaure = more energy stored in the same material. Simple as that. When you have a process requiring 400C for reasonable effiency, your storage actually starts to count from above 400C, so if you have 500C storage, you only have effectively 100C of usable tmperature difference stored in your bricks.
If it is cheaper to store at 1400 than 500, then that is an argument for doing so. Higher temperature is not a justification in its own right, absent economic benefit. It is also the case that conduction losses are proportional to heat, and it brings many other challenges as well.
