All of the internet was built on FOSS. If you want to understand what the world would have been like without FOSS, you need to look not at the internet, but at the competing technologies that were duking it out at the time. Novell Netware for the bare backbone and Compuserve/AOL for services. Keep in mind that these services charged per hour for usage. Even when we talk about grass roots systems like BBSes (the vast majority of which were FOSS, BTW), the task to connect them up together was ridiculously expensive. I remember paying $4.50 per hour plus 1 cent per bit transferred to interconnect my BBS. It's one of the reasons I switched to Usenet and UUCP email as soon as I knew what it was.

Back then there were gatekeepers who made sure you paid to play. The internet broke that because it allowed anybody to connect for free as long as you found a way to do it. The gatekeepers used proprietary protocols and software to lock you into their paywall. The internet used FOSS to make it easy for you to connect. That was literally the difference.