The question becomes: what is tx throughput at N billion users? What are the scaling strategies that will get us there? It is zk-rollups (or zk-zk rollups)? Is it sharding? Is it moving to custom hardware circuits? I suspect it will be some combination of all of the above.
We don't know what the answer is yet and we will devote tremendous resources to figuring it out. I don't want to give the impression that MobileCoin as it is written today will scale to Alipay levels of tx throughput, but I do believe there is a path to get there that requires a ton of work.
Does that answer your question?