I totally get where you're coming from John, but for me not from old age(although I'm not too far off that problem) .. but for me it's more to do with physical issues due to the many accidental encounters over the years.
Wrists knees, ankles .. well apart from one or two bones, I can't remember any that haven't been broken.
But the D750 is 200 grams heavier than an EM1-II ... so the D750 wouldn't be considered a 'camera you couldn't carry all day' in any real sense of the term.
That you choose not to carry one all day, is a totally different mind set, and I think a lot of hyperbole is allowed to propagate on the net with respect to the difference in weight between some gear!
eg. D750 with a typical consumer zoom lens isn't a lot more weight than most similarly designed mirrorless camera with a similar consumer zoom lens.
Where I concern about my long term future is that I can't hold a small bodied camera 'all day' .. and I mean camera in hand all day .. not camera around the neck or over the shoulder all day as most folks tend to do.
I can't do that kind of photography(metaphorically speaking). none of my camera have straps, I don't like them, primarily for the danger they can impose, but more importantly (psychologically for me) is that I prefer the camera to hand at all times.
smaller cameras can't apply tho. D70s was always too small, D300 was OK .. D800 nigh on perfect. I can hold the 1kg all day, that is other than a few mins at a toilet break, camera in hand for about 12+ hours.
No way I could do the same with the D5500 I recently got my daughter to play with. Camera body too small, no GRIP to speak of, and the fact that it's 100g less in weight than an OMD-EM1-II(at 400 gr or so) .. makes no difference to me. I simply can't hold it.
I don't have huge hands. Just my preference.
Funny side story: many moons ago I had the opportunity to shoot a nail into my finger, so I obliged without thinking about it. It's one of my auto modes .. if there's a way to inflict harm to self, I've probably done it!
So anyhow, large 4" nail in right index finger(I'm right handed). Hospital thought it best to place my entire left arm in a plaster cast! (for a finger!!).
Cast made camera holding a right PITH(pain in the hand .. literally) not cause of the finger, but of the plaster cast. Couldn't bend my right arm properly to hold camera.
Result was that I cut the cast, and made myself a finger splint out of it's remains. strapped it to finger when not required(ie. showering, dishes and photographizing).
moral: sometimes to achieve our desired outcomes, one simply has to push their own self imposed envelopes just that little bit further.