That was what I was wondering about when I bought my Macbook Pro in 2015 - but I'm still using it. I have to run 2 VMs plus Xcode before the RAM becomes a noticeable problem, and even then it's perfectly usable in practice. (Switching between VMs is not instant, but the very fast SSD means I'm not waiting for too long.) I think a quad core CPU would be a much more useful upgrade than more RAM.
And come to think of it, it was also what I was wondering when I bought my desktop PC in 2012, and I'm still using that, too.
I'm not saying it's unreasonable or foolish to be concerned - far from it, given that the RAM can't be upgraded - but I still do wonder when the tipping point for this will actually be reached in practice. During my formative years, you were doing well to get 3 years out of a PC, and now here I am with a laptop that's that old, figuring it will last at least a couple more years...
They will have a new machine for you to buy in 2 years time where the SSD doubles up as RAM and VRAM meaning you will have a gigabyte of RAM. Furthermore, since they will have an ARM chip in them by then you will be glad to see the back of your x86 legacy machine.
It just feels like 16G Ram (the max for mac laptops) is going to be too slow in a couple years or so.