I bought a Mac Pro in 2008, 8 GB. It's snappier then ever, mostly thanks to an SSD I put in this year. Upgraded the video card using a € 75 second hand PC card in order to use a 40' 4K monitor.
Also have a 15' rMBP late 2013, 16 GB. Being a 20+ years Mac user, I think this is an interesting thought experiment.
Running Skype, MS Office, 10 internet tabs and bittorrent will not be a problem on El Captain and it's very likely this will also be no problem on the OS that comes after that. The use of Office may shift to cloud more, but undoubtedly many users will still demand a version that at least also works offline.
Also, you don't mention that you want to run the latest OS so basically you could stick with 10.11.
I don't play MMORPGs, let alone in Parallels, so this I can't judge. I think virtualization is not very suitable *TODAY* for many games, then again, MMORPGs may be quite light and Parallels Desktop runs more smooth than ever on the 2013 rMBP. If you want this, my opinion is that 8 GB might not be enough in 5 years. On the other hand: If it works speedy enough for you now and you stick with El Capitan, you could keep using that.
Personally, I'd always go for 16 GB, because like you, I intend to use my machines for as long as possible and more RAM is one of the keys to make that happen.
Realistically: Laptops are more vulnerable, less sturdy build then desktops. I'd be more worried about whether your laptop will last 5 years of intensive usage than whether it will still run what you want it to now. In 5 years your software needs will probably be different.