Government mismanagement with a slide into global economic implosion and strategic thermonuclear war would do it.

I just don't see much else that can threaten the Cryptocurrency ecosystem as a whole.

I'd put the risk of fragmentation of the Internet into multiple hostile blocs with either fibres cut or Great Firewalled at 30% by the end of the decade. (Some countries already have systems close to this, and not just those you hear about). This will cause network forks and fragmentation, but is unlikely to stop cryptocurrencies on its own.

Internet is KYC, but encryption and VPNs get around it, and are too hard to put back in the bottle without completely trashing the economy (SSH with TLS can easily make a slow but secure point to point VPN to anywhere with access to a better VPN. Detecting and blocking TLS means no secure mass public internet commerce or communication).

Many countries already punish participation in cryptocurrency transactions with stiff prison terms, and they haven't succeeded in stamping it out even domestically. But they can prevent people running nodes. Unless people use Tor or I2P.

A global effort of the sort China wages against Tor seems unlikely, but Apple-style closed ecosystems could be tightened to restrict adoption of both VPNs and cryptocurrencies.

Finally, there are government PR attacks on crytocurrencies, and network-enabled attacks on the protocols or their implementations. But they do that already, and the ecosystem adapts.

Yeah, cryptos are looking pretty solid to me!