Actually in this case I think it still does not deviate from the original purpose.
I agree with this, it might happen that OP is more expose where Bitcoin is being traded in a centralized exchange but nothing in fundamental changed in Bitcoin. Which means that if it happens that centralized exchange fails at some degree, bitcoin can still operate and maintain it's value because it is more likely to operate according to original purpose. There is no middle man or institution in between, only P2P.
We dont need to be too idealistic someone who will only consider bitcoin as a medium of exchange because after all if you look at the current conditions , bitcoin is actually becoming more flexible and even sometimes it is not uncommon for people to consider that bitcoin is only for investment or as one of the trading assets . Thats not wrong because after all it is a condition that occurs now even though there must be a little corrected from his understanding but the fact that currently occurs that bitcoin can indeed be used as an investment or trading.
So,by looking at this we dont have to consider it a mistake because even though bitcoin is functioned as a payment option but with the flexible nature of bitcoin now coupled with the value of prices that are getting better in terms of prospects, IMO its not a mistake if this is considered a good enough asset to be used as investment material and reap the benefits in it .