There's not gain of function. The WIV1 virus already binds to hACE2, putting another spike on it and finding that spike also binds to hACE2 in vitro doesn't enhance its function at all. It does tell you how much that spike binds to hACE2.

You can debate if that's a good idea or not, but SARS-CoV-2 is way too distant from WIV1/SARS-CoV-1 to arise from any lab experiment with chimeric viruses. And its also too far away from RaTG13, which still needs decades of evolution to turn into SARS-CoV-2.