ASICs generally aren't flexible enough that you could simply emulate another controller in firmware, while FPGAs suck too much power to use on commodity network adapters. Writing a new driver (or bringing an existing neglected driver up to scratch) is going to be quicker than trying to make hardware that's compatible enough to work with a driver written for another vendor's controller.
(Besides which, as that other driver is probably maintained by your competitor, do you really think they're going to make an effort to ensure that their later updates are compatible with your clone controller? You'll still have to maintain your own fork.)
I have often wondered why there isn't a vendor-neutral register-level standard for Ethernet controllers, along the lines of AHCI and xHCI. There is the virtio networking standard, but as it's designed for VMs I assume it does not cover Ethernet link management. I seem to remember that VMware tried to promote a common interface for SR-IOV virtual functions at one time, but that didn't get very far. Again that would not have included link management.