25Gbps is where the transducers are landing for simple, single fiber capacity. 10Gbps underutilizes the hardware. 40Gbps does not look feasible without 2 or more fibers, it is an implausible step up for transducers any time soon.
In a data center with millions of connectors it makes sense to aim at the optimal feasible point which seems to be 25G for the next few years. It is a good idea to get all the parties interested in buying and selling at this performance point to agree on compatibility.