Not sure of what the actual discussion is, but if someone happens to buy 1 BTC in 2015, then it is still equivalent to 1 BTC in our today's world no matter the price assuming the person never sold. The unit, which is the sats never changed, but the price did. 1BTC as at 2015 is same as 1 BTC in 2025, but the price can never be the same.

I tried searching for the actual Price, but not sure of how the correct data is. In 2015, it made an ATH of +$500, and ATL of +$155. Subtracting the ATH from today's price is quite decent.