SCTP, for one, doesn't have any encryption.
Good, there is no reason to bind encryption to transport layer except to improve reliability of the channel in the face of active denial (e.g. TCP RST attack).
I disagree. To me there's at least one really compelling reason: To push universal encryption. One of my favorite features of QUIC is that encryption is baked so deeply into it that it cannot really be removed. Google tried to eliminate unencrypted connections with SPDY, but the IETF insisted on allowing unencrypted operation for HTTP2. I don't think that will happen with QUIC.
But there are other reasons as well, quite well-described in the documentation. The most significant one is performance. QUIC achieves new connection setup with less than one round trip on average, and restart with none... just send data.
Improvements to TCP helps everything layered on top of it.
True, but TCP is very hard to change. Even with wholehearted support from all of the major OS vendors, we'd have lots of TCP stacks without the new features for a decade, at least. That would not only slow adoption, it would also mean a whole lot of additional design complexity forced by backward compatibility requirements. QUIC, on the other hand, will be rolled out in applications, and it doesn't have to be backward compatible with anything other than previous versions of itself. It will make its way into the OS stacks, but systems that don't have it built in will continue using it as an app library.
Not having stupid unnecessary dependencies means I can benefit from TLS improvements even if I elect to use something other than IP to provide an ordered stream or I can use TCP without encryption and not have to pay for something I don't need.
So improve and use those protocols. You may even want to look to QUIC's design for inspiration. Then you can figure out how to integrate your new ideas carefully into the old protocols without breaking compatibility, and then you can fight your way through the standards bodies, closely scrutinized by every player that has an existing TLS or TCP implementation. To make this possible, you'll need to keep your changes small and incremental, and well-justified at every increment. Oh, but they'll also have to be compelling enough to get implementers to bother. With hard work you can succeed at this, but your timescale will be measured in decades.
In the meantime, QUIC will be widely deployed, making your work irrelevant.
As for using TCP without encryption so you don't have to pay for something you don't need, I think you're both overestimating the cost of encryption and underestimating its value. A decision that a particular data stream doesn't have enough value to warrant encryption it is guaranteed to be wrong if your application/protocol is successful. Stuff always gets repurposed and sufficient re-evaluation of security requirements is rare (even assuming the initial evaluation wasn't just wrong).
TCP+TFO + TLS extensions provide the same zero RTT opportunity as QUIC without reinventing wheels.
Only for restarts. For new connections you still have all the TCP three-way handshake overhead, followed by all of the TLS session establishment. QUIC does it in one round trip, in the worst case, and zero in most cases.
There was much valid (IMO) criticism of SPDY, that it really only helped really well-optimized sites -- like Google's -- to perform significantly better. Typical sites aren't any slower with SPDY, but aren't much faster, either, because they are so inefficient in other areas that request bottlenecks aren't their problem, so fixing those bottlenecks doesn't help. But QUIC will generally cut between two and four RTTs out of every web browser connection. And, of course, it also includes all of the improvements SPDY brought, plus new congestion management mechanisms which are significantly better than what's in TCP (so I'm told, anyway; I haven't actually looked into that part).
I'm not saying the approach you prefer couldn't work. It probably could. In ten to twenty years. Meanwhile, a non-trivial percentage of all Internet traffic today is already using QUIC, and usage is likely to grow rapidly as other browsers and web servers incorporate it.
I think the naysayers here have forgotten the ethos that made the Internet what it is: Rough consensus and running code first, standardization after. In my admittedly biased opinion (some of my friends work on SPDY and QUIC), Google's actions with SPDY and QUIC aren't a violation of the norms of Internet protocol development, they're a return to those norms.