the only reasons adding RAM to a machine in recent years has had any meaningful impact on its lifespan are
A - manufacturers have historically been ludicrously optimistic (and downright miserable) in their base RAM configs, leaving plenty of scope for users to add more of the stuff and get a significant performance gain in return.
B processor and graphics cards have gained so much headroom over the requirements of the base OS and basic apps, that they (thankfully) no longer needed a simultaneous corresponding upgrade too.
Instead you could just rip out the stingy 512MB or 1GB the manufacs had saddled it with and stick a much more useful 2 or 4GB that they should have shipped it with in its place. Instant upgrade.
Ten/fifteen years ago the old favourites were you could never too fast a processor, too big a graphics card or too much RAM, and considering what we had back then, even for folks doing basic tasks it was generally true.
But for folks doing general day to day task on a computer these days (as opposed to video editing, high end gaming, music etc) two out of those three are no longer significant issues.
And with 8GB installed on base models, then 'you can never have enough RAM' may no longer be an issue too.
Or to put it another way, when folks sitting with 8GB RAM need to reach for 16GB in order to extend the useable lifespan of their 2014 bought machine, its not going to be down to the requirements of their 2017's base OS and applications updates.
Its going to be down to a shift in the sort of uses they're putting the machine to. Uses that are going to need a whole lot more than just additional RAM shoved into the system to get any significant improvement in what it can do.
apple themselves has doubled the base ram in imacs every 2 years and why should that trend change?
there will be plenty of life in this machine and its parts when the ram is at its limits
but this kind of limitation, as much as i hate it, is fine in $500 machine or less but this machine isnt cheap. it isnt low end in price