As a classic owner...
Not overly concerned.
retina spec is good enough, only thing i'd need to upgrade is storage which you can remove and replace on retinas anyway.
I have gigabit ethernet. Used it a couple of times.
I have an card slot i've never used
I have an optical drive i've used a handful of times
However I also have
Heavier weight
Bulkier enclosure
No retina screen
worse battery life
Noisier fans
20 years of dealing with computers has taught me that by the time you need/want to upgrade a machine, there's so much that is behind the curve, you may as well just replace and pass your machine on to someone (or something) with lesser requirements.
Unless you bought totally the wrong spec in the first place, but that's not an expansion problem. That's just buying the wrong product.
Sure, some people buy lower spec and expand themselves without paying the apple tax. I did myself, but that's a whole different problem. That's not a device problem, that's a pricing issue.