1: Benefits cap - you say there isn't, I say there is.
You are wrong - There is a maximum monthly amount ANYONE can receive under the present formulas (BECAUSE there is a maximum annual amount that can be taken/contributed).

1a: If you remove the annual collection cap, you MUST remove the monthly benefit cap, or you are just stealing money from people.

2: Beneficiary vs contributers - you say they can never reverse
You are wrong - You are basing your assumption on constant raw population growth. You are leaving out MANY factors that place both physical (eventually you run out of room to put people), historical (developed societies ALWAYS experience reduced average birth rates, and population buldges), and medical limits. Your assumption is flawed, and the ACTUAL POPULATION NUMBERS AND RATIOS conclusively prove otherwise.

3: Your math

Wrong again - your formula is flawed because you are elaving out all the expenses of running the Social Security (and Medicare) department (which is high), but even if you were correct, it still doesn't work in the real world. Even if all we need is 3 workers per retiree, we do not have them long term. 3 workers per retiree means every family MUST have 6 children (3 for mom and 3 for dad). The population of the US would ahve to tripple every generation. Look around at all the people you know, how many of them have 6 kids (or more)? Out of all the people I know, I can think of 3 or 4 families that size, most are much smaller.

It IS unsustainable under the current model, and it always has been.