They have a reasonable sized moat then no? I’ve no idea $77bn seems large, but they intend to be the world’s financial system in 10-20 years so could be worth even more.
Do they? I'd rather own a couple of major exchanges in today's financial system for the same price.
ICE net income $2.1bn market cap $63bn
NDAQ net income $0.5bn market cap $23bn
CME net income $2.1bn market cap $72bn
CBOE net income $0.4bn market cap $11bn
You've listed exchanges, but Coinbase is much more than just an exchange. They are also a broker, as well as bank (provision of loans), custody-provider, etc.
And for the technical aspect, did you spend a few billion dollars when you tried? Potential coinbase competitors may.
I don't think a valuation higher than NYSE and NASDAQ combined (or CME and CBOE combined) is reasonable.