Cloud Mercato tested CPU performance using a range of encryption speed tests:
Cloud Mercato's tested the I/O performance of this instance using a 100GB General Purpose SSD. Below are the results:
I/O rate testing is conducted with local and block storages attached to the instance. Cloud Mercato uses the well-known open-source tool FIO. To express IOPS the following parametersare used: 4K block, random access, no filesystem (except for write access with root volume and avoidance of cache and buffer.
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Our study showed that the R7i and R7iz instances, based on 4th Gen Intel Xeon Scalable processors, offer the highest performance for MongoDB workloads. Moreover, the R7i provides the most favorable performance-to-cost ratio among all the instances tested.

I understand your explanation, but the reason why AWS took the decision is not clear, as the equivalent ARM instance type, m7g, has m7gd instance types, so is not only a matter of "consistency across different instance types" In the documentation / presentation of the m7i family there is not a single word of why they remove the built-in storage, and kept it on the ARM "equivalent"