What specifically makes "professional jurors" better, though? I won't say the idea isn't appealing, but what about making them professionals makes them better than our peers? There are already a bunch of heavily regulated professionals involved in the process so it's kind of arbitrary to claim that we need to regulate more of it.
Why not limit witness testimony to professionals too?
There are many ways for juries to just get things wrong, to misunderstand their role, to misunderstand the law, and so on. Sometimes these errors get recognised and corrected, at a cost; sometimes they just plain don't. Some jurors simply don't know how to pay attention and keep track for extended periods of time. Some education and training in advance could create massively more competent juries.
Why not limit witness testimony to professionals too?
Because we don't have a time machine to place professional witnesses where they need to be in order to witness things to testify about.
Sure, but we already know false/confused testimony is a massive problem, so is it necessarily the case that limiting it to professionals would be worse? Professionals analyzing the evidence, video tapes, etc. We're already moving towards universal surveillance. Many states ban one-party phone/in-person chat recordings, but if we allowed one-party recordings anywhere, any dispute over conversations could be resolved with personal recordings.
In the jury I sat on, we had a question about the the law and it's wording. The response we got was, "you have already received your instructions." Gee, thanks. Someone more well versed might have better understood the definition.
One of the juries I served, in the UK, had a similar question about a specific aspect of the law related to the trial. There was a 30 minute recess and judge, prosecution and defence discussed how best to explain - presumably without influencing in favour of either side - then we were brought back, told, and asked if that was enough. It was surprisingly involved for what we thought a simple clarification.
Having been on the inside of juries now, it restores some faith in some aspects, but you also realise far clearer where potential problems and limits are.
Why not limit witness testimony to professionals too?