I wouldn't say the BIOS was the weakest link. It was really the only obstacle, albeit a weak one. Surely, the BIOS was clean-room reverse engineered very soon and after that, the PC-clone market just exploded.

However, if there were no BIOS, the thin hardware abstraction layer that the BIOS provided would be part of MS-DOS. I see only two historical alternatives from that:

1. Microsoft would have had an even greater upper hand in controlling the PC market.

2. IBM could have kept the BIOS proprietary (even though as a part of MS DOS), which prevented Microsoft from selling MS-DOS independently with an IBM PC abstraction layer.

However, even if option 2 prevailed, Microsoft could have created its own BIOS to ensure that software written for MS-DOS would be compatible across the PC clone market.