Why not?

The currency is digital. If the current smallest unit is worth too much, then we can just increase the decimal places.

Bitcoin is not like gold where its limited supply makes it infeasible as a currency for a large number of people because the size of the coins for everyday purchases would be too small to physically handle (hence the concurrent use of less value dense silver).

Bitcoins can be split down to 8 decimal places.

Given the current loss/mining rate its totally possible for BTC to become difficult to operate, but its also possible for the mining cap to be removed or for coins to be split further.

8 decimal places are already bordering insufficient: there are real world currencies whose unit costs less than 1 satoshi. See https://www.fiatmarketcap.com/

Yeah, but in how many of those currencies are people actually using base units for transactions? I'd wager they're all trading notes denominated with thousands or millions of units for a loaf of bread, like the Zimbabwe trillion dollar note, http://news.bbc.co.uk/2/hi/africa/7832601.stm

Also, there is already a trick on the lightning network to trade sub satoshis without requiring a hardfork, probabilistic payments, https://courses.csail.mit.edu/6.857/2017/project/7.pdf

The currency is deflationary but you don't have any fundamental issues until you have issues subdividing the currency, but given that the protocol supports dividing down to 0.00000001 BTC that's not a significant issue (and I'm sure the community would patch that long before it became a problem).

Completely sustainable, because coins are very highly sub-dividable.

Same as the rest of the coiner nonsense

I don’t see why unchecked and uncontrollable deflation of a “currency” wouldn’t be sustainable.