No fork necessary. We can already use millisatoshis on L2.
If Bitcoins were the dominate and only currency, people would continue to buy food, clothing, luxury goods etc. b/c they need or really want those things and will be willing to trade bitcoins in exchange for those goods. Practically, Bitcoin is unlikely to be the dominate currency allowed in countries to participate in transactions without incurring significant taxes, so likely people will store wealth in Bitcoins(or other cryptocurrencies) and then convert to fiat currencies as they need to make their purchases.
In a pure BTC environment, the borrower is employed and being paid a salary in BTC. Loans would work just like in an inflationary environment, except now the overall deflation rate of BTC would be taken into account during loan origination.
Bitcoin is not a good asset to do loans in right now as it has high volatility, but as ownership of Bitcoin becomes more and more dispersed, then the volatility will reduce. Once the volatility of the asset is stabilized, then the depreciation rate could be factored into loans.
https://bitcoin.stackexchange.com/questions/1899/if-the-econ...
Fiat can, most of the time, grow or shrink the money supply as needed for the most stable economic outcome.