Are you sure it need a license ? I were thinking that AXI usage is royalities free (https://en.wikipedia.org/wiki/Advanced_Microcontroller_Bus_A...)
Then the Wishbone one isn't "perfect" as it can't map memories which have latencies without performance penality. It would be oky for the data bus of the cpu with or without cache, but for the instruction, without instruction cache, peak CPI would be divided by the memory latency.
however, if there's no need for a license, then it would make it lower cost to build SOC with off the shelf AXI IP
I can understand that copyright exists in the documentation that describes AXI, but that copyright covers only their documentation and not your own implementation (or is some derivation claimed?). I can understand that trademark might exist in the name "AXI", so don't use that name. Trade secrets seem to be moot, if you're not associated with whoever is keeping the secret. If none of the above apply, what basis is there for demanding a license?
It didn't use to be this way, but around the 80s the big precedents for this.
Most of the time, if you were a licensee before the expiry, then you continue to be so post-expiry. If however, you were not, then you have no existing contractual obligation, and can use it.
However it is something you need to consult a lawyer on, before making any decisions, because this area of law is more complicated than most, and is growing in complexity.
Usually royalties do go down after expiry in contracts.
Contracts also usually cover a lot more: Support, documentation and other guarantees.
This means there's a few businesses out there, that even after patent expiry still choose to renew their license.