That’s a difficult one to classify as there’s no set period of time wherein something would be classified as retro (as you said). Personally, I’d say the PS2/original Xbox and earlier are retro consoles. The PS3/Xbox 360/Wii are on the precipice, but not quite over the hill yet. Give it another few years and they’ll be seen as “old tech”.
EDIT: I think everyone has a skewed version of what retro actually is. I hear retro and think anything that hadn’t progressed to 3D graphical capabilities; but I also forget the PS3/Xbox 360/Wii are all almost 20 years old!
I got mine today, and whilst the layout takes a small amount of time to get used to, I find it pretty intuitive and easy to use. Obviously it’s not for everyone, but as someone who regularly loses the battle for the TV, this is hands-down a great purchase for me. Otherwise I’d have to use my phone, which is annoying as I have to disconnect my controller and connect it to my phone, and the screen is too small, or my laptop which is a bit too cumbersome. The Portal hits the sweet spot for me, it may not be for everyone, but I’m part of that niche that will use it regularly.
EDIT: My other gripe is the lack of being able to play through the cloud - I hope this is something to be added in the future as it’s literally just a WiFi controller with a screen.