That's easy to say 15-20 years later. When Chris was developing Poseidon, this may not have been so obvious for him, and also his focus was forward, towards MorphOS.
Look, this is all speculation. Whether this was obvious, whether it was not, whether it was intended or not. I do not know. The fix was done at the wrong end - it is too late for a change.
Unlike you, he couldn't use OS 3.1 sources as reference.
For this, you do not need any sources. You only need to know the documentation of IND_WRITEEVENT. Actually, the docs say it all: It sends the given event down the stream of input event handlers. That is it, and the source for the input.device do not tell you anything beyond this. For reasons unknown to me, Chris seemed to assume or require that it also creates synthetic keyboard repeat events, but it is not promised by the interface at all. And actually *should not*. If it would, it would break software such as FKey or commodities in general as they may create more events than intended, and different events than intended.
Anyways - you were around too, so why didn't you do anything...
You seem to believe that you have the right to demand service from me. How come? So, let's straighten up things for you again: I am not your AmigaOs service personell. First, I did not have Poseidon back then when Chris wrote it. How should I possibly be responsible for something I did not write, I did not use, I did not own and I was not even aware of. If he'd asked, I would have told him, but this is all years ago. We are doing something for it right now as the problem was reported, and that is the earliest point at which a problem can be handled.
The "kludge" has been the fix for the last 20 years or so, if OS 3.1.4 input.device comes with a better "solution", then fine... guess I could give it a whirl, the software license be damned.
The input device of 3.1.4 is the same as that of 3.1, and the only new thing it added was NSD. It does not provide any better solution than 3.1, but as of 3.1, there was already a correct way of doing it, namely create synthetic keyboard events yourself for keyboard repeat. Why Chris did not do that is unclear to me. 3.1.4 did not offer any particular "please fix Poseidon for me" feature.
Poseidon sources are available, I don't see what would disallow you to fix things "properly" if you so wish.
One word: Time. Poseidon sources for AROS are available, which do not compile on a 68K classic system. Poseidon sources for 68K are licensed by iComp, and from there I'll try to arrange an update. But anyhow, you can certainly try to be helpful and fix this in the AROS branch.