Honest question, how come? You can maybe argue something bout inflation, but it's not a very strong argument since a. the effects of extreme monetary easing aren't fully understood and b. there are countries which have been printing money with limited inflation.

Long explanation:

If you build a country around a sufficiently deflationary currency then it will stagnate over the long term because keeping your money yields better returns than investing or running businesses.

Let's say you run a company that buys $800 worth of ingredients to make a product that sells for $1000. The problem with physical reality is that you first have to buy before you can sell and the sell date may lie very far into the future. So what happens is that you will do the regrettable act of spending money first and once your product has hit the shelves of a retail store you have to sell your products for $900. Let's say you are a lone carpenter for ease of argument and you are running the entire business yourself. Those $100 are your income. Incomes of workers are going down over time.

In short what you are doing is sell low and buy high all the time because of physical reality.

This can get really nasty, to the point that if you have product stock that isn't selling at all you can actually make a loss! Lets say you have a table that only sold after 3 years for $700. It clearly wasn't worth it so why do it again? Even if you wanted, at some point your business will go bankrupt if you keep doing losing strategies.

Why is this a problem? If you were using Bitcoin to save for retirement then there will eventually come a tipping point where your Bitcoin are worthless because you cannot exchange them for services or goods anymore because the real economy is gone or not big enough to support your retirement.