I agree that it probably can't be completely stopped. A lot more can be done, though, and making excuses for people who buy gil is not helpful. Less RMT is better than more.
The shouting is a symptom of the problem, and it's a minor irritation compared to the direct negative consequences of RMT. It's difficult to stymie it without rooting out the cause. Preventative measures like the /tell filter are fallible, as we all know, and restricting /yells by means of the methods mentioned here hurt legitimate players too. We need to go for the origin.
I fail to see where it is a necessary evil. If the players who are supported by RMT suddenly disappeared I'm sure there would be some disruption because, unfortunately, the current state of the game is that some of the top-tier players are in that position due to RMT. RMT dependency is co-morbid with tool abuse, so some of the "Best" (read: most effective cheaters) would be gone. Stuff would get more difficult for some people who depended on that. But without them, the game can continue to develop properly, and we'll all be in a better place.
I reserve judgement on your point that RMT that aren't allowed to act without impunity might resort to other tactics, as I have no evidence, but if that is the case, that's why we need the STF on point - that's something that needs to be worked on actively.