-The fact that nobody is really interested in buying bitcoin to mainly use it as a currency aside from illicit goods, but more as a wild musical chairs speculation instrument.
-Scalability issues (and their solution proposals bring other problems and controversy)
-Mining centralisation, 51% attacks, the possibility of mining cartels.
-Higher fees in the future. BTC has low fees now that miners are allowed to print money mine bitcoins like quazy, but fees will be higher as block reward is reduced.
-slow transactions (you can't replace cash with something that can take half an hour or more to confirm, 0 confirmation transactions are not secure)
-ridiculous volatility that fucks up uses for bitcoin as a currency.
-the fact that merchants "accepting bitcoin" just create more selling pressure. Spending bitcoin = dumping bitcoin.

And countless others.

The idea behind the blockchain (a distributed ledger) has potential, sure, but the bitcoin blockchain and especially bitcoin THE CURRENCY are way too limited, in my opinion.

Centralised finance works just fine and while it might use the underlying technology/idea behind the blockchain, it doesn't need a new artificially limited, volatile, pyramid scheme-like currency.

For all we know:
decentralised ledgers = the internet
Any cryptocurrency like bitcoin = pets.com