In this case cassettes and chains are the same age as they all date to the date of 10sp conversion, but good point.
Perhaps I misunderstand something here? It’s not the age of the components but their state of wear that matters. Unless you use both rear wheels (cassettes) equally – and maybe even then! – one of them will cause chain-skip before the other. Tom_e’s method of changing the chain early may partially solve this (at a cost), but it doesn’t solve the other problem: the need for different chain lengths (number of links) depending on cassette. At the very least, ideal shifting will require faffing around with the B-tension screw.
All of this sounds like hard work to me, but many are made of sterner stuff than me.
I have one bicycle and wouldn’t want it another way. The machine is a Spa Audax with a lightweight road-orientated build, but I often take it off road. On many occasions I have done 50-odd km on a canal path strewn with large rocks. It works. There is certainly no need for 45 mm tyres for “light offroading on hard trails”. Where compromise is needed, I usually lean toward optimising the road performance, since that’s where I (and most of us) spend the vast majority of the time, and where I’m most concerned about performance too.
Some compromises away from a purely road-bike layout have a negligible effect on road performance, e.g. rack eyelets, 36-spoke wheels, and 57 mm drop callipers.