Spain over Germany is a bit harsh I guess. From the Global Sporting Salaries survey in 2019, the most recent one I think, salary rates in Italy are higher than in Spain on average per player, when you exclude the 3 giant clubs, real Madrid, Barcelona and Juventus(who had the third highest wage bill in football that year).

That year, clubs with average per player salaries in GBP between 1.9 and 4 million were: 4 in Germany, 5 in Italy, with Lazio very close but just under, 2 in Spain. Spain had the poorest clubs at the bottom, followed by Germany.

The main reason I had Germany below those two however is because of how much Bayern dominates. In Italy you have 6 very competitive big clubs, in Spain you have 3, in Germany you just have Bayern Munich.

>well-developed top-flight leagues

It's not about this at all. The best developed leagues are England, Italy, Spain, Germany, France in that order. On top of that, you have different levels of inequality within leagues, for example huge in France(dominated by PSG),big in Germany(Bayern slightly less dominant) and Spain(a few big clubs) and small in Italy(lots of pretty strong teams) and England(strongest "weak" teams). A Spanish player will choose to play frequently at a top Italian club over sit on the bench at Real Madrid.

The other really big factor, the reason why Saudi and Qatar and to a lesser extent England have so many domestically based players is money relative to talent. Also why Brazil and Argentina who have popular domestic leagues have so many players playing abroad. Almost every league has some sort of cap for foreign players or quota for domestic ones. The exceptions are Germany and Portugal without quotas or caps and Brazil where there is so much talent there's very little need to ever import players. Asian leagues generally have very restrictive, 3-5 caps on foreign players. The gulf leagues especially have a lot of money. Thus, a decent domestic player is worth their weight in gold. In England, you need to have a certain number of domestically trained players, a hangover from the EU days as you cannot directly discriminate against players from other parts of the EU. There is far more money in English football than there is in European football, and as such a good English trained player, most of whom are English or British, is worth a lot to English clubs.

Everyone has seen those flyers. Prostitution is very prevalent, but it's not "huge sex slavery".

There are "China experts" everywhere you look. The qualification required to be one on Reddit or in a social circle is very low.

Okay, hold up, let's stop spreading disinformation here.

>policy finally caught up to them

The generation where the gap was the largest, after the introduction of the one child policy(50ish years ago) and before massive rapid orderly urbanisation which decreased the relative economic advantage of having boys(30ish years ago), has already "passed". You do not see sex number gaps in Chinese cities anymore than you see them in western european cities. Where the gap is present is in impoverished rural areas, but even then the bulk of that has "passed"; especially in those areas, the expectation is marriage at 25ish for men, men who are now 40ish.

>so that fuels a huge sex slave trade

China does not have a huge sex slavery problem. According to the walk free foundation, focused on modern slavery, less than 0.1 percent of the Chinese population is enslaved. That number is comparable to or better than Eastern European EU members, who generally are the best of the middle income developing countries in most things human rights. Compared to other developing middle income countries, China is actually very good at combating modern day slavery.

What China does have is a prostitution "problem". Prostitution is illegal in China, but tolerated. That system means that sex workers don't have the legal protections they should have. That is a million miles away from the situation you suggest is present.