No, you don't have to renew. They can stay here for rest of 2 years without insurance.
And next time they want to visit again for 2 years, just come in with 6 month visitor visa and apply for extension. They do not ask for medical insurance.
But, as mentioned by others, for the safety of your parents and your wallet, insurance is recommended.
If payment is too much for you, i suggest looking into high deductible options, instead of $0 deductible.
You pay lot less up front but in case you need use it, you pay $500~1000 deductible, which is nothing compare to cost of hospitalization without insurance.