But the a5100 will not be replace the big camera, more the Nikon V3 used mostly by my wife at the moment.
Of course it will, Bernard. Of course it's only for 'the wife'. We already knew that.
But
IF, perchance, you find yourself inadvertently reaching out for the 'little one', don't worry; it's just a subconscious reaction to the 'new kid on the block'.
When it starts repeating itself - still don't worry.
When you buy the adapter to put the Otus on the 'little one' - still don't worry.
When you give 'the wife' the D810 and keep the 'little one',
that's when you need to start to worry ...
M
[/obviously written with purely humorous intent]