Do you honestly believe that increasing supply would change anything?
Well let's see... one of the most infamous hoards in Global to date was the Pole Dance emote, exceeding 200 million meseta in player shops across Ships 1-3. Then it appeared in Throwback Collection and spent several weeks being available for 35-40 million and even a few months on I see it's only gone back up to ~75 million on my ship.
So you know what? I'm gonna go with yes on this one.
Limiting the prices is the only way to make it more reliable
What is "reliable" supposed to mean in this context? That sellers holding an item worth 200 million would suddenly decide "Oh well I'll just sell it at some 100 million cap instead because I like throwing vast amounts of money away" ? That's not a realistic expectation at all.
If a handful of players are holding onto an item that gets rarely put on the market at 100 million and snapped up immediately (which at that kind of value disparity would surely happen) they won't do the same. They'll hold onto their excessively valuable item to use in a trade, either for more money than it can sell for (to circumvent the cap, potentially risking scamming) or for another excessively valuable item they want down the line which is also too high in value to be wasted in player shops.
I mean, extrapolate the thought process a little. Imagine they add a market pricing cap that's stupidly low like 15 million. Do you think players who get something like a Rodeo emote (~20 million) or that backless sweater (~25m) from the newest scratch are just going to throw 5-10 million away in an inefficient liquidation? What if they wanted one of the other pulls? They'd trade for what they wanted instead and ignore the player shop entirely. In the process, most items would disappear from shops, and F2P players couldn't ever have a chance to get them as they do now. And you're saying that's a good thing? It's no different regardless of what your arbitrary cap is.
What those rich players you decry would do in such a situation is switch the focus of their hoarding to items that are below whatever your imagination is for a sales cap, buy them, and hold onto them only until their value rises until they hit the cap before relisting them. So even items that would have been cheaper prior to the instigation of this cap would be forced to become more expensive. And because they were cheaper in the first place, those rich players will be able to do it to a greater volume of them too.
This idea would accomplish the exact opposite of what the people calling for it want. It's not the only way to make anything more reliable; it's one of the worst ways to interfere.
Improving and fixing those problems would help, but the fact it's PvP means that it still wouldn't be popular.
Oh I agree; PVP in a game that's PVE is always a largely pointless endeavour because if people wanted PVP they'd just play a game that intended to be PVP to begin with.
But PVP in PSO2 is even more unpopular than it would be because of how unplayable and unrewarding it is. To put it into context: There are several other enjoyable quest types that players have nevertheless abandoned for not being rewarding enough compared to newer ones. Battle Arena not only isn't rewarding enough but at least half the players in every game come out worse than if they hadn't played. And the servers' tick rate and biased geolocation make it unenjoyable too. It needs way more fixing than other quest types they can (and often do) just slap a UH difficulty on before players will care.
But again this topic is about the myth that empty PVP says anything about the server's population. It doesn't. It just says that Battle Arena is a truly awful game mode.