Other coins can become the everyday "currency" coins you use to buy coffee.
Bitcoin works just fine as it is for storing and transferring relatively large sums of money/value.
High security is only possible because of high revenue to miners, which in turn is caused by demand. If you remove the component of the demand that is caused by people wanting to transact on a smaller scale, the demand decreases, the revenue decreases, and the security decreases, which can lead to even less demand and a downward spiral.
3 TPS does give us 3 x 31,622,400 = 94,867,200 transaction per year.
I do understand your argument. I do understand the demand side issues. I do not know of a way to prove it will be sustainable at 3 TPS so I guess we will just have to see how this great experiment turns out.
One thing we can both agree on: at 3 TPS Bitcoin will never be the everyday currency of large portion of the population.