Eventually in this case being approximately 18-24 months from the first wave, which coincidentally is very close to where we are in the current pandemic.

The Spanish Flu is a single data point. I wouldn't read too much into the 18-24 months stat.

The point isn't that Covid will be as fast as the Spanish Flu, it's that the Spanish Flu did not take a long time, and should not be dismissed as an example where it became milder eventually. In fact, we'd have to be quite lucky if covid became mild in the same time frame.