The problem with death costs was that it disproportionally affected those who were either not very good at combat or new to bossing. I.e. the money leaving the game was coming from players who weren't bringing in much money. The knock on effect of this is that players are less likely to continue PvM activities because they can't afford to keep dying as they learn the boss.
High level PvM may bring a lot of high value items and GP into the game, but those who bring in the most tend not to die very often, hence don't have to pay much out in death costs.
The Dev Blog states death costs accounted for 62% of all GP leaving the game. Reducing death costs therefore would have a massive impact on inflation. To help alleviate the impact, the GE tax was introduced.
The GE tax should not be seen so much as a tax. It is more of a convenience fee. The 2% flat rate is negligible compared to the natural price variation on almost every single item. Therefore, 99.9% of the time, the convenience fee has almost zero affect on the average player.
Your concern regarding much higher valued items, however, is a real one. It is thought that when the GP limit fix comes into the game, there may be limits to how much tax is paid on higher valued items. Like you say, pushing players to Player-Player trading can open more people up to traditional scams. However, we're already in that position regardless as these items cannot be traded on the GE right now anyway.
Also, the GE isn't completely infallible to scams. I won't be posting how they work here, but I have seen first hand players be scammed out of 100's of millions of GP via the GE.
In my opinion, the GE tax is the most fair, least discriminatory method of controlling inflation. More could possibly have been done to target the GP generators, such as alch machines, but there are probably other factors at play which made Jagex choose this direction.
I also agree that high rewards should have high costs associated with them.
~A~
29-Jan-2023 20:36:51