Is it possible to peg a cryptocurrency to a stock or bond index?
There are already cryptocurrencies that are backed 1:1 by dollar, see Tether and USDC, or even backed with gold, see Paxos Gold, and yes all of that are redeemable. there is no problem with it so far.
There may be some trust problem, are the issuer really back the cryptocurrency with real asset? (this can solved easily by external auditing though) At 2018 there are some issues that tether aren't pegged 1:1 with USD, the result is Tether value are decreasing to around 0.9 USD. Weirdly enough, even without an external audit(they never got an audit since that scandal), people still believe in tether and the price of tether jumped out to 1 USD Again.