Frames and forks defintiely trickle down, and quite quickly really, look at the carbon stuff you can pick up as a £1500 full bike now and compare to even 5-7 years ago.
Running gear definitely not though, I just picked up a 1995 steel trek as a runaround/town bike, it's got a full 1995 spec RSX group on it, kinda Tiagra level back then I guess, definitely under 105 anyway, and its still slicker shifting and nicer than the Sora stuff on my 2012 Trek that cost about 700 quid.
I'd definitely pick older Dura Ace/Ultegra (maybe even 105) over current Sora and Tiagra.
Sora last no less or longer than anything else. It is heavier for sure - no argument. But there is no 'or wears out quickly'
It really really does, the lower level kit is not as durable, in MTB or road groups, it wears out and gets sloppy a lot quicker.
There's a lot more high milage top end stuff still going strong than high milage mid range a entry level, whether looked after or not.