Must TCP use IP?
I haven't read the whole RFC but the language in section 1.4 seems to suggest that any "lower level" protocol can be used.
The interface between TCP and lower level protocol is essentially unspecified except that it is assumed there is a mechanism whereby the two levels can asynchronously pass information to each other. Typically, one expects the lower level protocol to specify this interface. TCP is designed to work in a very general environment of interconnected networks. The lower level protocol which is assumed throughout this document is the Internet Protocol.
Internet Protocol Suite
TCP is not short for TCP/IP.
TCP/IP is often used as a shorthand way of saying "The Internet Protocol Suite" and usually includes other standard protocols. When people say TCP/IP they are usually including UDP over IP (in which UDP is used instead of TCP) and a great many other protocols such as ARP, ICMP, DNS, SNMP and other application layer protocols.
Application Layer
Applications use Application Layer protocols such as SMTP (for email). These sit on one of two transport layer protocols - TCP and UDP. A few application layer protocols will use either or both of UDP and TCP but most are used with only one transport layer protocol.
Transport Layer
TCP and UDP are two transport layer protocols used in the Internet Protocol Suite. If there are others I don't know of them and any others would represent a vanishingly small specialist use. Others transport layer protocols have been defined - their usage probably represents only a small proportion of global IP traffic†
Internetwork Layer
Whilst it might be theoretically possible to use TCP over something other than IP, in practice TCP is always used over IP - the Internet Protocol. IP moves packets between networks (think of IP as connecting multiple LANs together)
Network Interface Layer
Ethernet is just the most popular family of low-level link-layer protocols on which TCP/IP is carried, but TCP/IP is also widely used over ATM and others.
Appendix 1 - A note on Transport Layer Protocols
The only transport layer protocols in significant use on networks that use the Internet Protocol Suite are TCP and UDP.
†Just for fun, I measured traffic on my (very) small LAN, which includes NetBIOS (over TCP), SSH, Rsync, Email, software updates, DNS, general Windows-box chatter and a few other types of traffic.
Note also this statement in Google's FAQ for their QUIC protocol
Why didn’t you build a whole new protocol, rather than using UDP? Middle boxes on the Internet today will generally block traffic unless it is TCP or UDP traffic
(my emphasis)
The reason why TCP/IP is such a common abbreviation (as opposed to, say UDP/IP or SCTP/IP) is because the two protocols were designed together, and in the original paper by Vint Cerf and Bob Kahn, the two concepts were combined together into a single protocol. Soon thereafter they were divided into IP to provide routing and TCP to provide flow control, multiplexing, error-detection, etc. It wasn't until six years later that UDP was introduced to provide a "lightweight" multiplexing layer without the rest of the overhead involved with TCP.
Still, TCP and IP are two separate things and completely and intentionally independent. The fact that TCP does not require IP is immediately apparent with the fact that TCP can run unmodified on both IPv4 and IPv6, which are two completely different protocols.
With a little work, you could create a competing protocol to IP that would serve the same purposes, but it would probably have to contain most if not all of the same features, and would probably end up looking a lot like IP anyway. You could argue that extensions to IP (such as IPSec) are effectively alternate layer 3 protocols, so there you go.