What happens when the remaining 104 unassigned protocol numbers are exhausted?
There is sometimes drama with it, though. Awhile back, the OpenBSD guys created CARP as a fully open source router failover protocol, but couldn't get an official IP number and ended up using the same one as VRRP. There's also a lot of historical animosity that some companies got numbers for proprietary protocols (eg Cisco got one for its then-proprietary EIGRP).
I do hope we'll have stopped using IPv4 by then... But well, a decade after address exhaustion we are still on it, so who knows?
It uses them a little differently -- in IPv4, there is one protocol per packet, while in IPv6, "protocols" can be chained in a mechanism called extension headers -- but this actually makes the problem of number exhaustion more acute.