Just a thought to bolster the theory that shardplate is live spren stuck in the form of shardplate. Remember the interlude with the researchers studying flamespren? They specifically point out that once measured or recorded the flame spren remained in the same shape. Once they destroyed the figures they wrote down the flamespren returned to it's baseline changyness (that's a technical term, you understand).
I'm not married to a particular method of plate being created. Perhaps it was a completely natural evolution of the Nahel bond. Perhaps it was an engineered creation by one group like the Bondsmiths (though given that there were apparently only three Bondsmiths and a lot of KR with plate they might find themselves a bit busy). Perhaps it was a collaboration between the Highspren and some engineering.
The point I come to is if part of the process of creating the shardplate (whatever that process might be) involved defining how the spren would act as shardplate...... "you will adjust to fit your wearer, you will absorb stormlight to fit yourself", etc....... then the plate could still be living spren which was defined into a specific shape/purpose.
I'm assuming the more esoteric functions of the plate seen in the flashbacks were because of the Nahel bonded spren interfacing with the defined spren of the plate. So while bonded spren may have been involved in the making, and the increased awesomeness of "old" plate, the plate's continued existence wasn't dependent on the bonded spren continuing to live. Think of it like a car. The plate is the body, the suspension, the wheels, the seats, etc. The bonded spren is the engine. Take the engine out and the car still exists, but now it needs a different form of propulsion. In the case of the plate it's the infused gemstones, vs the KR themselves fueling it with stormlight.
And that makes me wonder about something else. Did the modern users of the plate somehow add the gem slots inside the plate in order to power it, or were they already there? We know Kaladin can power plate directly based on the duel scene, however just powering even a helmet drained him completely in a few minutes. Perhaps the KR ran the plate with the gemstones, and they topped them off as needed. Just a thought.