Sure it may sound pessimistic, or conservative.The common cold use to kill humans many millenniums ago, it does not today. The flu was much deadlier as well. I could make a list a lot longer then yours, but since those viruses have become so benign it is hard to remember them all. Most have gone off our history books. Only the few scary ones are still with us.
Look. Neither of us know the future precisely. You made a valid guess, and so did I. Maybe we should leave it to the future to determine which guess is more correct. I just doubt we will need a vaccine every 3 or 4 months. That sounded a little pessimistic to me.
But it's also based on the data.
1. We know that COVID19 antibodies fade in a few months.
2. We have not been able to create a human vaccine for any other Coronaviruses.
It seems unlikely that a vaccine will last long if the antibodies fade quickly.
It would be a surprising coincidence that despite decades of effort, the first effective Coronavirus vaccine just happens to be the one for COVID19.
As far as "the flu", yes the more lethal strains tend to die out. Because they kill their hosts.
The most successful viruses are high spread, low symptom.