It was/ is being done, many users including myself have already made the migration.
The key issue is how to you make it “mandatory”? Nobody at ECC or ZFND has the users private keys to force users to do anything.
Also, It looks like there is about 100K ZEC sitting in the old Sapling pool. If the balance on that pool ever goes negative (ie: someone is trying to remove more than was put in) then the Nodes will automatically reject the transaction. That means users who still have “real” funds in the pool will be out-of-luck because a hacker “got out the door” first. But the overall supply integrity of 21M would remain intact.
This has been discussed at length in other threads, suggest reading through them: Zcash Counterfeiting Vulnerability Successfully Remediated - #68 by joshs