Well, yes, the AAIB do voice opinions on maintenance. The recent fatal Sandown PA28 crash, with the well trashed engine, is one recent example. But the CAA does nothing about this, evidently, which makes the AAIB's views worth next to nothing.
Deciding what to change is often tricky. I (N-reg) could run a lot of stuff "on condition", and with a "suitably chosen" maintenance company could tacitly allow the ticking of a lot of boxes representing work done but not done. Unfortunately, planes are not like cars which can be fairly safely run into the ground without the owner giving a damn. The owner does have to get pro-active and has to understand which bits are important and which should therefore be done, even if not legally necessary.
And this applies equally on N-reg and G-reg, because the "box ticking" culture is widespread, and EASA will not change anything there.
A while ago, one American chap in the Socata user group wrote that he found a crack in his prop hub, and didn't like the cost of fixing it. It turned out his prop was 1985-original and never been looked at. I don't think 23 years on a prop is wise, but where does one draw the line? I average 150hrs/year which makes a £3k prop overhaul about £3/hr, which is hardly significant, along the engine fund of £10/hr. I got mine done at 6 years, not least because it was full of nicks (flying to airfields with !!!!!ty runways covered in stones) which had been individually dressed out.
As already stated by others, this thread is a bit of a puzzle because the owner in this case has claimed to be extremely rich and to fly a huge number of hours, yet the 700hrs probably does represent a 6 year prop overhaul - is there an earlier overhaul period? I though the 3 year one is gone and it is now as per the mfg, which is normally 6 years.
I have done two mag overhauls. One at 500hrs and one at ~ 700hrs when the engine was rebuilt for SB569A. Each one turned out fine, and the 2nd one was done in the USA (with the engine) and cost hardly anything which is why I had it done; to make sure the engine started off as a new known quantity. With a single shaft dual mag engine, this is definitely the weakest link in the engine...