I'm not sure I got what you meant but judging by the photos, you are comparing them in store conditions and not even with full white image. Also camera makes difference but Air in your photos looks brighter and the fact that older Air has TN screen than changes a lot depending on the angle you look at it.
Anyway, for sure 2017/2019 Pro is brighter than Air 2018 and that is not very much brighter than many other middle/low range computers but my point was that how many actually keeps their brightness settings at 100%? Because if you run both new Pro models and Air at the same brightness levels the only difference between the two is colour gamut and contrast ratio (black appears darker in Pro if you have white level adjusted the same on both).
Again, if your main use conditions for the computer are outside sunny day or as bright conditions inside as in Apple Store, then you may need Pro.
Just for fun, I measured some brightness difference between my computer screens. My desktop with Dell IPS monitor measured just about 395 nits, My Zenbook IPS was about 315 or so, my iPad 2018 about 420 nits and finally my new Air 2018 about 350 nits. Slight variation in colour temperature between all of them makes each appear a bit different for naked eye. However, none of them are set anywhere near the max. brightness when I normally use them. For example my desktop Dell is at 40%, my Zenbook 40-50%, iPad round 30-40%.