Overall, reasonably well-balanced
Fog cloud will work how people think it works
I can't tell you how many times I have seen people suggest using fog cloud as cover against enemy archers. Even Treantmonk lists it as a benefit of the spell in his popular guide to 5e wizards. With your proposed change, it will actually work as expected in this and other cases. Particularly-bizarre cases are also fixed. RAW, you are far more likely to hit a target 600' away with a longbow if you (or they) are obscured by fog.
Higher AC creatures benefit more
Your proposed change inflicts disadvantage on attacks by most creatures within fog cloud or darkness. The higher a creature's AC, the better this change is for them. The following examples illustrate this. (Critical hits are ignored for now.)
- An AC 16 wizard is hit 50% of the time by an orc with +5 to hit. With disadvantage, the wizard is hit 25% of the time. Disadvantage caused them to take 50% less damage.
- An AC 21 fighter is hit 25% of the time by an orc with +5 to hit. With disadvantage, the fighter is hit 6% of the time. Disadvantage caused them to take 76% less damage.
Furthermore, a larger percentage of the hits on a high AC character are crits, compared to a low AC character. With disadvantage, crits happen 1 out of 400 times instead of 1 out of 20. This benefits high AC characters more than low AC ones.
In my experience, PCs - except those in the back line - tend to have higher AC than their opponents. A first level fighter with the Defense fighting style can have AC 19, rising to AC 21 once they get plate. Thus, the PCs will usually benefit more from the changes than their enemies.
Sneak Attack is worse
RAW, you can Sneak Attack someone in a fog using only the adjacent ally rules. Now, you'll also need to hide or get Help to cancel the disadvantage.
Advantage-generation is better
Various spells, Reckless Attack, and the Help action are better with more disadvantage in the mix. Since PCs tend to hit more than they miss, it is usually better to cancel disadvantage than it is to gain advantage.
Wind wall gets slightly weaker
Wind wall is only really useful when defending or assaulting a fixed position, where your own ranged attack capabilities are much less than the enemies'. You may be able to peek out from the side of the wall, shoot, and then take cover behind it again.
With the proposed changes, fog cloud is a reasonable substitute. At 3rd level, it provides a whopping 60' radius of fog, covering your approach for around the same distance as wind wall - 120'. Fog cloud only provides disadvantage on ranged attacks, while wind wall causes them to miss entirely. On the other hand, fog cloud also blocks almost every non-AoE spell cast by most creatures.
Warding wind is largely unchanged
Warding wind only affects those around you, so your allied archers and casters can still hit foes that aren't engaged with you. It also hedges out fog, which may become more prevalent with your proposed changes. These differences help it maintain its niche.
Tanky PCs will want fog cloud to make them more evasive. Control casters won't want it, since it blocks their view. Wind wall and warding wind will continue to have niche use cases. Using the term "realistic" to argue about D&D 5e is a losing battle, but the proposed changes add a degree of verisimilitude and tactical depth without disrupting the overall balance.