I have to again say I don't think that end-game gold is even relevant. Most significant expenditures in this game are one-time, and it's not a struggle to get to turning a profit even at the earliest points. (In my current game, I'm on week 3, day 1. I have 15k gold, I am 60 xp on a slave in my party away from being able to complete a slave request for another 6k, and my main character and the first guild apprentice have nearly all sixes for factors. I've dumped at least 40k gp into factor upgrades across my team. I just paid 1k in interest and got informed that my hard mode non-forgiven next payment to the bank is 3k gp. I'm not sweating it. My best service character is also my best wizard, and she still only makes that 26 gold a turn doing group, or she makes 110 gold a turn on fishing or cooking.) This leads to the "it starts to pay for itself once it's too late to matter" problem. The early parts of the game are the only parts where there is any difficulty, and charm is basically pointless then. You already need to use phys and wits just to play the game, and it's also what all the jobs you actually need to progress the game are based upon.
Service only gives gold, but gold is only worth what you can buy with it. Most things you actually want are either available in quantities more limited than your gold or are gated by things other than gold. It's very easy to quickly hit a point where you can't find anything you actually want to spend gold upon. (Even if you just buy resources like leather, the supply is limited to something like 6 per day, and several necessary resources aren't even for sale until late-game, if ever. Construction slots are another notable bottleneck, and upgrading takes phys, anyway. I don't keep one or two characters constantly chopping 30 lumber per turn/120 wood per day because wood sells for a lot, I do it because I need 50, 100, 500 lumber per upgrade, I'm doing 2 upgrades a day at this point, and there's only 40 rough wood on the market every day! Also, the g/LU value of most resources actually drops as you go up in quality, so high-quality goods are cheaper than just making low-quality stuff and selling it, then buying the materials you can buy - you only need to actually work for the materials you can't buy.) This inefficiency of just making gold, again, means that you really need to make way more gold than other methods of making money (and still early enough that it matters) for service to even start being relevant, like making 200 gold per turn with a 500 value, 100 charm character. (Although I again think the entire value system needs rethinking...)
Likewise, you can't just hire a service worker that can even make back her own salary, (she makes 1 gold,) you specifically need to do extensive training to level up a character, almost certainly through grinding dungeons, on a character that has to take classes useless for combat, so it's a tremendous drain on the player's time to get a "payoff" only measured in useless gold you can't even spend anymore. The most valuable thing in this game is not gold, but trained characters, and to get to the point where a sex worker is viable, you need to have already spent a lot of time and effort building effective combatants and laborers, so once you've invested in setting up something that gives you everything you need, what's the point in spending even more effort in doing something even more tedious to set up something you clearly don't need? Just to make bigger number show up? When you already make number go up infinite high just punching "x4" next to "next turn" over and over?
Hence, even if you can make charm theoretically viable for exactly one unique character through using the broken royal and pumping thousands of gold and lots of training to make her start to pay off, you've clearly already gotten to the point where you can spend thousands of gold and weeks of training in-game time and a couple hours of real player time on a vanity project that is an albatross around your neck until complete. If you've played to the end of the game either not using charm or service or finding it a total waste of time every time they tried, why would players be motivated to try doing this, if they even remember it exists? (You know, like recruitment...) If you do this, it's not because it makes you money, it's just because you can, and because you apparently really like seeing princesses suffer.
I remember this also being a problem in Strive for Power - money is too easy to make too many other ways like "just set someone to hunt and someone else to merchant," especially with all your off-duty combatants which have no drawbacks. What reason is there to suffer negative consequences for "big payouts" in gold making the extreme versions of sex workers that can actually make more than a merchant, (especially when the most "profitable" options ruin a character or involve ruining your reputation,) when you're already swimming in gold like Scrooge McDuck being legit?
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Honestly, thinking about it, one thing that the "host trade meetings in your mansion" idea could do is just make more trade goods available on the market. Again, there's just way too little you can actually spend your gold upon, so it just sort of congeals in your vault with nothing you can use it upon.