Actually, that's not a bad thing. UDP is small enough to have nearly no overhead, but complex enough to let firewalls do their job. Six of the eight bytes in its header would probably be in the header of any transport layer protocol anyways (only the checksum might be unnecessary).
Wikipedia lists over 100 assigned IP protocol numbers [1], and while it would break existing firewalls, adding a new protocol would certainly require less work than the transition from IPv4 to IPv6. But UDP is already simple enough that there's very little benefit in not just building on that.
For example, IP routers often peek at UDP/TCP port numbers to calculate ECMP flow hashing. This is technically naughty but it's read-only and it's only an optimization that isn't required for correct forwarding.