Except it won't make a difference between two phones with the same battery capacity and hardware. By your argument, this will last shorter than a Galaxy S7/S8/S9 which also had 3000mah battery because it's screen is brighter. Which is false because the screen isn't actually used at a brighter level, just on the 3min overdrive mode. On regular use it will last close to any other phone with the same battery capacity and comparable chipset.
A bright display only affects battery more than a dimmer display if you actually set both on their max brightness. A G7 user who keeps his screen at 300nits brightness will have the phone last about as long as say, any other SD845 phone with an LCD screen at 300nits, even if the G7 can reach 1000. The max brightness of the screen doesn't matter in taking expected battery life into account because the phone can't actually use the max brightness it's capable of. Also some reviewers already noted that on regular use, the G7's screen actually doesn't get brighter than other phones on normal usage where there's not enough UV to trigger superbright mode, but max brightness would hover around 500nits like everybody else, meaning it won't matter.
Sure more brightness=less battery life, but only if you were actually able to use more brightness. But you're not, so the impact isn't as big as you make it out to be. The actual optimization of the software will have a bigger effect.