I have 8gb in my early 2015 13" rMBP and currently have open, with no issues according to the memory pressure:
Safari with 6 tabs running
Mail
WhatsApp
Messages
Excel
Powerpoint
Streaming from Apple Music via iTunes
View attachment 804931
For light to medium use 8gb will be perfect for most.
It's interesting to note you're using about 1.5 GB of RAM in Safari alone, with just 6 tabs. You also have a 1.5 GB worth of memory compression.
Meanwhile, I currently have just 5 windows running and have over 2.5 GB of RAM used in Safari alone. One of those sites is using over 1 GB by itself, and I've occasionally seen some sites use as much as 2+ GB worth of RAM. It's quite remarkable just how much RAM various websites are using these days, and it continues to creep up over time. Now, imagine those who keep >20 tabs going at a time. We're talking potentially 4-5 GB RAM just for Safari.
Yes, 8 GB will work, but it often requires lots of memory compression, and if you decide you want to run a lot of Safari tabs/windows while multitasking, the amount of memory used can be remarkably high. And if you have more than one user active and/or if you have a VM or two running, then the strain on memory is going to be even more and you run a strong risk of having to swap. Also remember, this is just with 2018 usage. It gets worse every single year. Back in 2009 I bought my Mac with 2 GB RAM, and it was OK as a baseline for light usage. I don't think 2 GB back then was as good as 8 GB is now, but nonetheless, 2 GB back then was probably equivalent to somewhere in between 4-8 GB now.
By my estimates in the past, RAM usage goes up something like another half every 4-5 years. That means roughly say 10% a year. With my example of a 2 GB Mac in mid-2009, that means approximately 5 GB in 2018. That's probably in the right ballpark. Now, let's say you are a light to moderate multitasker who likes keeping Safari tabs in the background, so you actually need about 6-8 GB. That means in 4 years you'd probably want between 8.8 GB and 11.7 GB.
So no, in four years you won't need 16 GB. You may not even need 12 GB, but ideally you'd want more than 8 GB, and this is what I've been saying for just about forever. You shouldn't look at this as you needing 16 GB. You should look at this as wanting more than 8 GB in a few years, that is if you keep your laptops a long time.
And finally, this is not even considering the previously mentioned benefits of application caching in memory. Caching of bloated apps really speeds up app launching of those apps. Even on my PCIe SSD endowed iMac and MacBook, loading up Office 2016 is really slow, unless it's been previously cached in memory.