If 1/10 of the supply is being wiped every 3 years then we have a problem cuz SP tokens will only be backed by 0.1 steem.
This seems like a misreading of some aspect of the economics involved. What are you basing this off of?
The money supply roughly doubles every year, then every three years it is reduced to 1/10. This keeps the coin count numbers approximately within the same range longer term, but doesn't affect much else.
Huh it doesn't affect much else? SP tokens will no longer be backed by 1 steem instead it will be backed by 0.1 steem which is a big deal imo.
3 years later it will happen again and 1 SP token will only be backed by 0.01 steem. You end up in a situation where there is a lot more SP token in circulation than steem , then starts the cash out race !