These are just variables. Variables can be changed.
So do you claim that block reward can also be changed? Is it also
just a variable.
There is a difference.
Change of block reward would create a HARD FORK of Bitcoin. It is the most "sacred" variable of Bitcoin, it cannot be changed without hard forking and disagreement in the community.
Changing block size OR default fee will not create a hard fork.
The only REAL limitation is the speed of the internet connection and TFLOPS of processing power.
And the speed of anonymous internet connection, in case someone want to make bitcoin fully anonymous. And the cost of it.
And what is stopping you from using a LITE client over an anonymous connection ? Why the hell do you need a FULL client over TOR for ?
Also, there is a network which can handle such traffic - I2P.
So why the hell people keep shouting "Bitcoin is not designed for microtransactions" ?
Because it's nonsense to store sub dollar tx in thousands of full nodes around the globe and propagate between them. The cost of bandwidth and storage is simply to large to make it economically feasible.
We have (or we will have) tree pruning for that. Not everything has to be stored.
And hard drives capacities & internet connections speeds will be improving over the years.