I think that the comparison of a lot of fiat currencies to in game GP is a good one - while they may be store of values in the short run, or at least appear to be so; in the long term their value is heavily based on trust in the game to continue to function.
The problem isn't trusting the game to continue functioning, the real problem is that nowadays there are a lot of bots that heavily farm on multiple accounts the in game gold to dump it for couple $ cents/million.There is also risk of inside jobs, and this actually happened recently at Jagex where a moderator have stolen from players their GP.All these factors and other similar ones has lead the price of GP to go downhill, which answers the question,- no, it's not a good store of value.
I disagree. What the problem boils down to is still that there is a central entity/entities that are capable of modifying people's GP balances arbitrarily, as they wish from a case to case basis. Whether that comes in the form of keeping the game functioning, or having to trust the moderators doing their job properly.
Again, the comparison to fiat is that its centrality. Moderators and game developers in RS are essentially the regulators and the central bankers in the fiat world, if you want an analogy.
It is because of this that it is not a good long term store of value, not necessarily because of gold farmers. A loosening in the monetary policy, or an increase in the frequency of gold drops would result in debasement of the currency instantly, just like inflation in real life. If the game was decentralized, then even with gold farmers, people can be sure that inflation will be controlled and there won't be modifications to rules in the short term, which means stability in the long run.
On a side note, who would have thought that RS would turn out to be such an interesting economics analogy?