My friends and I largely don't carry cash, and even though I typically carry a single 20 and a 100 dollar bill for emergencies, it's hard to split cash unless someone else can give you change, which requires them to carry a bunch of 5s 10s 1s (coins?).

Paypal fails like I said because of network effects. More people in my friend group have venmo than paypal (even though paypal owns both), so when it's time to pay, venmo is the default.

Nothing is inherently wrong with paypal, it's just that nobody in my group uses it. So, just plain old network effects.

People don't carry cash as much as they used to, because they don't need to, because apps can do it.