I'm a little confused as to which point you got yourself confused at, then made up stuff to fill in the blanks?
Now, I see your name is son-of-satire so maybe that explains it, but for the sake of anyone reading, I'll have at it.
There is no common sense reason why multiple waves can't get bigger. I think the confusion lies in the fact that the virus can move around the country, yet the country counts the total as a singular thing.
For example, the north of a country can be highly infected in wave 1. Then, later, the much more populated southern half of the country gets infected when the north has already had its wave and most people have decent immunity. Unfortunately, the south is more populated, so the second wave, even though the north is not counted more than a handful, is much larger.
Then the extremely populated East gets hit, the north opens up from lockdown, the south ignored the lockdown rules totally, and 2 weeks later you have yourself an even larger third wave, because, like I said, as a country, you count the North, South, East and West as one lump total. (presumably, the west followed rules without complaining about freedom and so 0 people got infected in this example)
I hope that helps.
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To address another point, there is no absolute proof immunity is long lasting. In fact:
It's not known if people who have only mild symptoms, or none at all, will develop a sufficient adaptive immune response.
The immune system's memory is rather like our own - it remembers some infections clearly, but has a habit of forgetting others. Measles is highly memorable - one bout should give life-long immunity (as the weakened version in the MMR vaccine does). However, there are many others that are pretty forgettable. Children can get RSV (respiratory syncytial virus) multiple times in the same winter.
Then, comparing to other Coronaviruses:
Four produce the symptoms of the common cold and immunity is short-lived. Studies showed some patients could be re-infected within a year.
Research at King's College London also suggested levels of antibodies that kill coronavirus waned over the three-month study.
What you're doing is building a conclusion based on what is little more than an old-wives tale / poorly done educational system, based on something that 'seems to make sense' - the idea that once you get infected, you can never be infected again. Which is just not true.
Herd immunity can work in small pockets for diseases that do go away after one time, like chickenpox parties creating herd immunity among the local kids. But how come chickenpox is still around? Because it spreads easily enough to be able to hop around the country quietly and slowly enough to continue infecting the uninfected for years to come without major government intervention (because it's not deadly if you get it as a child).
Unfortunately, CoViD is deadly, it's not seasonal, it's global, and it's something you very, very much do NOT understand in any sense of the word; logically, scientifically, or politically
RE: WTF is a COVID Third Wave?