Sadly the state of the internet means that only TCP and UDP will be widely used for the foreseeable future. There are a quantity of old routers which just throw away protocols they don't know, and they don't know much. See https://en.wikipedia.org/wiki/Protocol_ossification

SCTP is deployed more widely than you think, all mobile network(3G/LTE/5G) use SCTP for varies signaling protocols like S1-MME

Yes but no. It is used in operator networks but that isn't on the Internet.

I think you might be mistaken on the flexibility of the infrastructure.

I personally don't think IP (tcp and udp included) will be used on Internet 2.0. Theres just too much baggage with those protocols.

Perhaps we'll see Internet 1.5 leverage encapsulation and build a huge underlying canvas for new protocols to blossom, but we seem to be approaching the point where anything with an IP address is too much of a hassle to maintain and a longterm security liability/commitment.

Just moving IP to the borders of a network would open up space for more secure protocols.

Its probably time we put the middle one behind port 79 anyways.

SCTP also lives on in the WebRTC spec for data channels, so all (modern) browsers also implement it