Agreed. If you understand the resource requirements of your environment then vertical scaling is totally viable. Ive seen lots of scenarios where teams shoot themselves in the foot because they fail to just think critically for a second about their deployments.

Consider a Redis-based cache: For future proofing you could introduce a sharding scheme that deterministically allocates keys based on cluster size. That would be operationally pretty complex. Not to mention also introduce complexity in edge case scenarios (e.g. your cluster size is transitioning).

Alternatively, you could do a little back of the napkin math to figure out what the size of your keyspace is going to realistically be and just...allocate a machine that fits your needs up front.

Obviously we could make arguments about budget and all that, but those are optimizations that are better to defer.