BMW V12 VI, 1926

BMW V12 M70, 1986

BMW V12 N74, 2009

What you're seeing is increased use of plastic in each engine as time progresses - not only in the very obvious shrouding covering them, but components in the engine and its ancillaries. It's a case of what's suitable for each use. Cost is a consideration of course, but as modern materials improve it's not always the case that the traditional metal is better and plastic is only chosen down to cost.

We know that metal to metal interfaces can cause issues too, especially when the components are different metals. Ref the changes from Cal 11 to 11-I to 12, or the troubles McLaren have had this season in affixing an aluminium wheel nut to a titanium stub axle.

None of the movement manufacturers were designing or making movements to last 50-odd years, and we arguably see the quality of even the metal movements declining with time as they become more mass manufacture items. Watchmakers can extend the life of a movement by making parts in metal, which is more difficult for plastics true, especially where the composition of the plastic is not known - plastic is a catch all term after all, covering a huge range of materials.

So, whilst we might be disappointed to see so much plastic in the 5100 and a bit peeved that it's all hidden dial-side in a bit of sleight of hand, it's not right to knock it just for being plastic - modern life as we know it wouldn't be possible without the material.