When you purchase some mutual funds units at Zerodha, the units are delivered to your CDSL demat account that you have with us. To debit any units from a client’s demat account the Broker needs a POA from the client. Without the same, we will not allow you to place a redemption request. You can refer to this link to know more in regards to POA.
After 15 years once you give an instruction to redeem the mutual fund, the units will get debited from your Demat account and the AMC will give a payout of the units redeemed in your Name to us as the orders are being directed through us and we will credit the same to your trading account from our end.
In a worst-case scenario where you feel that “zerodhas financial condition is not that good and if Zerodha is not able to transfer the funds to your bank account due to XYZ reasons”, if at that point of time you feel that your investments are unsafe with Zerodha, before placing a redemption request you can easily transfer the units via DIS slip from the CDSL demat account you have with zerodha to some other demat account or you can even rematerialize the units and transfer them to the AMC and place a redemption request.
You can also refer to this explanation where Nithin has explained clearly in regards to how safe are your investments with zerodha 
hope this helps.