Your biggest expense will be people. Rails is ancient at this point, but I regularly see people ship product with it in fractions of the time. You can also re-write parts in rust later, when you need to.
We should all be so lucky
Usually what happens at scale is that the SQL database starts slowing down as more data is loaded in. Partitioning, indexes and painful refactorings aren’t prioritized. Engineering will champion “a faster language”—that incidentally allows them a new data model where they add in proper indexes at the start. They aren’t really incentivized to realize they could see the same gains by just improving their existing data inside the Rails monolith.
Source: experience improving performance (including latency) with multi-billion-dollar Rails monoliths