I think you may have misunderstood the GP and are perhaps misusing terminology. You cannot meaningfully scale vertically to improve write availability, and if you care about availability a single machine (and often a primary/secondary setup) is insufficient.
Even if you only care about scaling reads, eventually the 1:N write:read replica ratio will become too costly to maintain, and long before you reach that point you likely sacrifice real-time isolation guarantees to maintain your write availability and throughput.
Even if you only care about scaling reads, eventually the 1:N write:read replica ratio will become too costly to maintain, and long before you reach that point you likely sacrifice real-time isolation guarantees to maintain your write availability and throughput.