Decentralization is abstract. 
But control is concrete.
Who is in control of a digital system?
Can someone delete you?
If they can, it’s centralized.
If they can’t, it’s decentralized.

But most people never get deleted. Composability is more concrete.

You secured tickets to Taylor Swift on TicketApp because you own the “Top Listener” nifty for listening to her for 5+ years on MusicApp?

Couldn’t happen offchain.

Unfortunately this scenario needs some network effects, but will happen.