Theory: Ishar, possibly with the help of some other heralds and spren, created the first Shardblades and laid the foundation for the Nahel Bond, Ideals, and the Knights Radiant.
Basically, I think that the Heralds were looking into better tools, weapons, and tech to help their people survive the desolations. Someone had the bright idea of mimicking the Honorblades (because its not like someone trying to imitate soulcutting powergiving indestructible swords hasn't happened before in known Cosmere history - looking at you, Vasher). Ishar either knew how Honorblades worked or he figured it out, and realised that if certain spren could theoretically manifest in the physical realm, it could work (Why these spren, though? Maybe because Honorblades were made up of a chunk of Honor's investiture. If this new weapon was to be made in some sizeable quantity, then such chunks would need to be found in nature like any other resource. Cue, sapient spren.) But since said spren were actual, living, thinking, sapient beings whom Ishar had to convince to become tools of mass destruction, Ishar had to devise a method by which they themselves would choose who they bond with, with this chosen person then being their pathway into the physical realm. Also, such power was too much to simply give away to anyone, since it is implied that the humans destroyed their previous home with surgebinding, and the Heralds being privy to this information meant that they would input some sort of check. One more thing to note is that since much was lost in the desolations, the Heralds would also be looking to pass on some sort of law or codex through between desolations, if only to keep the Humans firmly following Honor and Cultivation, and not fall under Odium's influence again. All of this culminated into the founding of the Radiant Orders. Perhaps it was also Ishar who made the spren promise not to bond with the Parsh, since this would give the strength of Honorblades to the enemy as well.
I like this because it requires Ishar to have certain characteristics that he is implied to have in the books anyway, and the implications are vast. For example, it means that the Surges and surge pairs are actually seperate from the Knight Radiant concept. As in, there are other ways to access the surges, which the Humans once knew about and probably used to destroy Ashyn. Also, since WoB states the Honorblades were prototypes, it could be that it was Ishar and the Heralds who actually created or proposed creating Honorblades to Honor.
The theory is also corraborated by the fact that atleast one other Herald (Nale), considered Ishar the authority on Knight Radiants and the Final Desolation.
If this theory is true, it means that the Knights Radiant were an answer/solution by the Heralds to multiple problems at the same time. They were a weapon against the enemy, a means to make the people aspire to Honorable ideals, an extension of the Herald's reach across Roshar and even between desolations, and a knot that tied mankind and sprenkind together against Odium.
It also explains why the Parsh would think the Humans could give something to the spren that they couldn't. They were aware of the fact that changing forms involves attracting spren. From what they could see, the Humans were also attracting spren and gaining something in the process. Since these spren were expressly forbidden from going to the Parsh, and given the differences in their physiology and capability (like how the humans couldn't hear the Rhythms), and given how the Human-spren bonds manifested differently compared to Parsh-spren bonds, it would not be a big leap to assume that the spren got something from the humans that the Parsh simply could not give.
Some questions: How much does Jasnah know of this, since she has spent time in the cryptic homecity? Are there leaders among the 9 radiant spren types (discounting the Bondsmith Godspren) who have a list of symptoms to look out for to recognise an approaching desolation and send out their people to look for and bond with prospects? And how does the Oathpact factor into this?
Sources:
Edit: I'm going to try and explain how this works in my head in a different way. My guess is that the Honorblades were made by Honor making a connection to the Heralds, and then pushing a chunk of his Investiture through this connection into the physical realm, manifesting it as a blade. One part in the design of this blade is that it can "connect" to the spiritweb of individual people, and through this connection grant them the ability to use the Surge-pairs. Another important aspect is that these blades do not always exist in the physical realm, but are held in the cognitive/spiritual and can use their connection to this individual to enter the physical realm at the individual's behest.
So when trying to imitate this design, one needed a chunk of investiture, preferably of Honor, and make it form a bond with an individual of a similar kind to that of an Honorblade. My guess is that certain communities of sapient spren already existed in the Cognitive realm, and Ishar went to them, and designed a method by which these spren could from bonds. And since becoming a shardblade was the spren putting themselves at risk of death, Ishar also gave them the power to choose whom to bond. Perhaps the Ideals came from this requirement. Perhaps the individual needed to be "broken" in the first place for the spren to be able to bond with them. And in this inital pact, perhaps Ishar also put in a requirement that the Spren not bond with the Parsh in this manner, as at this time, the Parsh were an enemy of Mankind.