However, I agree with
@BrotherSutek that just because these armies were less popular, it didn't make it right for GW to deliberately give these factions subpar rules and/or just avoid giving them decent ones
Am I understanding this correctly; is your theory that GW
deliberately gave unpopular armies weak rules?!

This is the problem with internet speculation, you've constructed this entire narrative without a shred of evidence. Saying that GW purposefully wrote weak rules for unpopular armies is a pretty substantial claim, especially when it is unsupported. There are so many problems with this theory and so many much more plausible explanations.
On the low end of the power scale, you are basing your theory off of a singular data point. Of the bottom 4 armies [Bretonnia, Beastmen, Tomb Kings and Orcs & Goblins], two are running old books, so their lack of power is much more attributable to that factor rather than your proposed theory. While it is a shame that some armies were not updated, we can at least understand GW's rationale and regardless, that is a completely different argument from the one which you have proposed. Next, we have the O&G, who are one of the most popular factions, so that actually cuts directly against your narrative. So essentially your entire argument revolves around the Tomb Kings. Drawing such a definitive conclusion from a single weak army is foolhardy and dubious in my opinion (especially when the O&G offer a one-for-one counterpoint against it).
A game with this level of complexity will never be perfectly balanced (look at T9A, they've sucked all the fun and life out of their game in pursuit of this mythical balance unicorn). If GW could wave a wand and have the game balanced without sacrificing the elements that make WFB so great, I'm sure they would (because a better game can only further help sell models). We're talking about a series of 12 army books, written across a span of multiple years and penned by numerous authors. Are you suggesting that GW throughout those many years instructed the AB authors of unpopular armies to write weak rules for them? Why? To potentially depress sales? And if so, I'm surprised not a single leak about this practice was ever released (as far as I know).
Not only that, but the accusation doesn't even hold true:
- O&G are extremely popular and they have weak rules
- Ogre Kingdoms are not that popular and they have really strong rules
- we have the absolute most popular armies residing in the upper, middle and lower tiers, and the same can be said for the less popular ones
@NIGHTBRINGER you say yourself that whenever you play your Warriors of Chaos against
@Mrs. NIGHTBRINGER 's Wood Elves, they stomp them - doesn't that bother you? Doesn't it make you concerned that the conclusion of such a game is already a foregone one?
That's a bad example, because a significant skill gap exists between us. On the rare occasion when me and the Mrs. play chess, she gets stomped there too for the same reason. Going further, same thing occurs when we play football/soccer. Discounting skill, both chess and football can hardly be considered unbalanced games.
On a side note, I actually believe that the Wood Elves, in skilled hands, should be considered top tier. They are a finesse army, which means they have an extremely high ceiling in terms of competitiveness, but require an experienced general to reach it. I see nothing wrong with this, some armies are easier to play than others, which offers players a range of play styles. There are many players that would probably stomp me and my WoC with their Wood Elves. Furthermore, there are actually army matchups in which the Wood Elves fare better than the WoC (i.e. against the other Elven factions).
I personally don't think Wood Elves are well suited for a newer or less experienced player. But in the right hands, they are downright scary!
I'm surprised the Mrs actually even bothers to play your Chaos Warriors if she knows she's going to lose to them - she must love you very much to keep on agreeing to do so.
I haven't fielded my WoC against her in a long time, because I find the games to be less interesting. She tends to lose quite significantly against the Lizardmen too, and the Lizzies are usually considered to be a fairly well balanced mid-level army.
As
@Lizards of Renown pointed out earlier, fielding the TK is a great way to help bridge the gap between players with a skill disparity. Our "best" games have come when I have played my TK. We'll see how she fares against my Chaos Dwarfs! (
insert evil Dwarvian laughter)