It's often assumed that without the ARPA the internet never would have existed. ARPA, however, is just one of many "internets" that sprung up over the years. There was also FIDOnet, Prodigy, BIX, Compuserve, MCInet, etc. Everyone with more than one computer thought to hook them together. Heck, even some friends of mine invented their own internet back in the 70's.
If it wasn't ARPA, it would have been something else.
The examples you listed arrived after ARPA. CompuServe, which arrived pretty early in the greater scheme, was founded in 1969 as a time sharing computer service. It wasn't until quite a bit later that it became a packet-switched computer network, likely in response to the success of ARPAnet's implementation.
In contrast, the plans for ARPAnet were finished in 1968, and the system was operational in 1969. The concept was pretty pie in the sky at the time, and it's likely that the success of ARPA led to commercial implementations of packet-switched networks.
Additionally, the early private networks did not have the character of the "public Internet" which is a crucial component of the Internet we know and love today. Indeed, the reason we care about SOPA is that the public Internet, and not a patchwork of private internets, won the day.
If it wasn't ARPA, it would have been something else.