Hmm interesting thoughts!
My take on this however is not so much that forms were meant to pass on the most important (or only important) techniques of a system, but were a specific learning modality or tool to communicate and instil in you the principles of the system. So forms are aspect which help tie all things together in a way, and therefore learning a form properly can assist your use of all kihon (fundamentals).
For example, something like the simple 180Ā° rotation from Taikyoku kata, drilling that endlessly really instills in your body the ability to squeeze your leg in as opposed to shove off and fall forward without control, to rotate on specific parts of your feet, to maintain great posture and alignment, keep your head level through movement, and develop back leg drive through the last half. All or most of those qualities can be immensely helpful to a roundhouse kick for example!
Someone then might ask, well can't you just practice the technique directly in order to get better at it? Of course, but the forms to me are about a holistic development of the person as a whole, and in HOW you move your body, how you transition, how you generate power in awkward "unrealistic" positions (if you can generate power or move efficiently from those very "unrealistic" and awkward positions, how awesome would that training be for developing your body as a whole to really move and do these things when you're not under perfect balance etc???). Developing an internal "quality of movement".
The 2x turn and hammerfist sections near the end of kata Saifa have nothing to do with a hammerfist. Tight, concise rotation, and learning to utilise relaxed heaviness through your whole body to DROP the technique through, an ability to pull through your frame like a cable via the elbow as opposed to focusing on hand movement.
I swear that my rigorous hours upon hours of kata practice have made my EVERYTHING so much better, my balance, control, understanding push/pull, transitions, sparring dynamics etc I personally think kata has helped how I move. Can I prove this or do I have evidence? Heck no! Just a strong sense going by how I move now.
So to me it's not so much that because they didn't include certain techniques within forms that those techniques are then pointless/irrelevant/not a part of the system, but that the specific forms act on multiple levels of exploration of the key principles which you can USE throughout the whole entirety of the system. Instilling a body intelligence that you may not necessarily get through basic standing kihon practice. And the forms don't need to contain every technique of the system. It's about reinforcing and deepening QUALITY of movement as opposed to quantity.
Just my random musings, don't know if that was off the topic haha.