That's not a valid question as it assumes that everyone will be running a pruned full node.
It is a perfectly valid question, because why should one *not* prune? Altruism?
Especially on a chain that adds 1 TB/year...
because if everybody prunes 100% the network will collapse - thats the reason why no one would prune all (yes this is a dillema, but as most people will run the "official" bitcoin-core its just a matter of good design - and that this works can be seen with miners: they would just mine empty blocks if they would be only interested in coins).
i'd say what would happen is that every node keeps about 10% of "other" transactions which would help bootstrapping in a real p2p manner.
DnD made a nice reply to me as i was having the same concern a long time ago.. sorry i cant find it anymore ;(