Like many middle aged people I recently re-discovered cycling after a 15yr hiatus this summer.

I was immeaditally drawn to a GT Sora, because I couldn't believe the price and how light the bike was. But a couple of cycling friends really discouraged me from buying it because it uses Shimano Sora components which are almost bottom of the line.
The Sora is still light years more advanced then my old Suntour Accushift. But I decided to restore my old bikes and ride them this summer.

I was pretty content until last night, when I went to a bicycle co-op. And they had a black shiny Ridley Fenix 7005. For an affordable price.
But when I asked the owner about how good the components are he kinda cringed. And said that they are 105. He further went on to say that this years, Dura ace becomes Ultegra the next year and so on. So the bike is about 3 years old.
I decided to do some research on the Ridley. In 2014 the Fenix with 105 is listed at $1999.00, in 2015 the same bike is listed at $1200. Does technology change that much that even a new bike is worth half?

TLR
Lets face it I'm never going to run the Tour De France So what does the difference in components mean to a avg person like me? Does having Sora mean it only lasts a year then wears out? What if I bought a 10 year old bike with Dura Ace, does that mean it's not as good as a new 105?

I'm trying to figure out what I need to spend so I'll keep the bike around for 20 years like my last one.