No they are definitely losing purchasing power.

Allow me to illustrate:

Theres two people in the world and $2. They each have $1

Person A invests $1 USD in monopoly money in 2010, Person B saves $1 USD in a piggy bank in 2010
Person A now has $6,000,000 USD worth of monopoly money in 2017, While Person B has $1 USD in a piggy bank
Person B wants to join Person A and get some monopoly money. So they buy $1 worth in 2017


Person B has 1 monopoly dollars and Person A has 6,000,0000 monopoly dollars out of the total of 6,000,001 dollars

Therefore, Person B has significantly lost purchasing power. In this case of only two people in the world their purchasing power would drop by a factor of 6 million times. The 6 million number is based on the ACTUAL price increase of bitcoin from 2010 to 2017. This example illustrates how phoney and absurd the price increase of bitcoin is