If something is worth a constant amount of dollars it's value is indeed not constant but declining. Actually declining quite rapidly as well.
It depends on the reference. If something is worth 10 dollars now, and it is fixed, it will be worth 10 dollars in 2 years, and it will be constant.
What you mean is that the dollar is inflating, and such the value of the object should decrease over time. Which is wrong.
The first and most severe dollar deflation was during the depression from 1818-21 when prices of agricultural commodities
declined by almost 50%.
The second was the depression of the late 1830s to 1843, following the Panic of 1837,
when the currency in the United States contracted by about 33%, a contraction which is only matched by the Great Depression.
It is not because you
wrongly believe that the dollar value will always decrease, that it is true.