All you are talking about is the present day though; when I got my MBP 15 inch with 4GB, at the time it was enough, but today it isn't.
Again, personal preference. My post aren't addressed to anyone person.
Your laptop is 6.5 years old. That is old in laptop standards, no matter who makes it. 6.5 years from now, Ram likely won't be the only thing limiting you. Has integrated graphics, and portable dgpu get more power, programs and operating systems will utilize them. 6.5 years from now, with a exhausted lithium battery (unless you replace it, not cheap) you laptop will be next to worthless, and will struggle with all but the more basic task.
As I mentioned above, you can spend a large sum
of sum of money and try to make your laptop last forever; there is nothing wrong with that. Alternatively, you can spend a reasonable amount, sale said laptop in 3-4 years, recoup a decent amount of the cost, and buy another reasonably price laptop. You might save money the first way, or you might save money the alternate way. If the two paths are close monetarily, the alternate path will provide the more enjoyable experience with the updated technology. Apple, or any company will take as much money as you can give them, it is up to you to make informed decision.
When I built my desktop, I opted for the gtx 1070 over the gtx1080. The price difference was $300+ at one point, almost as much as another 1070. I am not pushing a 4K monitor, so by the time the 1070 is showing it's age, I can get a $300 or so gpu that will be significantly more power than the 1080; and I didn't spend any more or less money. See the point?
Again, I'll say it, its personal preference. If you are doing something that all really needs 16gb ram, or 8gb is close to limiting, then go for it. But if you are trying to get a laptop to last 6.5+ years and not show its age considerably, while doing processor intense work; good luck.