The reason why I think even a single exit might do a lot to the price of MusicianCoins, is that financial success (not only in the music industry) seems to follow some sort of power law: a very small number of members amass a large part of the wealth [1].
Do people behave like this in real life (ie abandon their group when they strike success)? Well certainly not most people - men like Warren Buffett and Bill Gates haven't taken up citizenship in some tax haven to opt out of US taxes - but IIRC one of the Facebook founders did.
There are at least two reasons why it may be a bigger problem for MusicianCoin than it is for the USA: 1) the group size is bound to be smaller, so a single such event would cause a bigger shock, and 2) it's a much easier decision to sell your crypto-coins than it is to move country.
Of course this is all arm-chair economics on my part, and I actually hope you're right that in practice it's not such an issue - I love the idea and would love to see it work well.
[1]: see these sort of numbers, where the top 1% pay 23% of taxes: https://www.vox.com/2015/4/14/8406445/tax-state-local-distri... - and I reckon what would have been really interesting is if they had broken it down to how much of that 23% is carried by the top 0.1%