You're using a nonstandard definition of deflation. When the real economy grows faster than the money supply, prices go down, which is deflation. If the money supply is growing at 1% per year or less (which will happen in year N of Ethereum), it's easy for real growth to outstrip it. Alternately, if demand for ETH grows faster than the supply the price of other currencies will fall relative to ETH.

> That's why it doesn't make any sense to buy it now, unless you need it now.

I think inflation/deflation has very little impact on investment value, only on nominal price on a unit currency. A currency could be wildly inflationary and still a good store of value, if people sell them cheaply because of the expected inflation and it turns out future demand was underestimated. In that case, it would still increase in price just as much as a deflationary currency.