By this argument inflation is irrelevant since inflation's "negative impact is just based on a stubbornly held belief that currency is a benchmark of value".

By this argument all the "benefits" of deflation are an illusion.

The observable fact is that the adjustment from mild inflation to mild deflation is painful. People lose jobs because both employees and employers do not reduce salaries fast enough and loans are paid off in fixed amounts and end up sucking more resources than expected to pay off and therefore reducing investment.

The ironic thing is you are right that, theoretically, the value of money does not matter and is quite arbitrary. It's the unforeseen changes in the rate of change of money value that causes problems. Steady is best. You can write contracts (loans and employment contracts) that take a steady %2 inflation or a %2 deflation into account.

So, given that we were at %2 inflation, why not just leave it there since contracts already take that into account? Why call for unneeded pain by calling for switching to a currency with deflation when the benefits are an illusion?