I think the reverse is true. New bike sales have been in decline and this is thought to be due to two factors; the buzz created by Bradley Wiggins et al is or has reduced. Secondly many who threw themselves in to the sport have been selling very good bikes they no longer want.
I have many cycling buddies who have picked up absolute bargains in such circumstances when in the past they would have bought new.
That's interesting - hadn't considered the possiblity of demand / interest falling in general. While this is a shame I guess it's not so bad if it's more a correction following a surge of public interest rather than a drop from a more stable long-term norm
Extras or upgrades rarely add much to the achievable price of a bike and can detract from it.
Most second hand buyers are doing so because they cannot afford new.
Extras and upgrades take the second hand bike further away from new in their eyes, which makes such a bike less desirable.
I'd agree that upgrades don't tend to add a lot of value; in addition to the originality argument I guess a lot of upgraded components are wear-parts so you don't know how much longer they'll last anyway. In addition I think many only upgrade once the original bits have worn out; raising further questions about the amount of use the bike's had / the extent to which it's been maintained.
I think the argument for used bikes isn't just that people can't afford new ones; it's also that you can get so much more for your money due to the depreciation. For example my £450 got me a "used" but never-ridden Tiagra-equipped CFRP road bike, while new it would have only stretched to Sora on an ally frame; which a year down the line would have dropped in value to maybe £250-300 and further south after that.
All this news doesn't appear to have reached the Croix De Fer sellers though.. There's a 2015 CdF 20 on ebay up at £500 starting bit and £750 BIN.. admittedly it's apparently low mileage and has a few extras but the BIN is insane considering an extra £250 would get you a brand new example of last years model with much better components. The massive gap between what the seller expects and what people are willing to pay is illustrated by the fact that even with a starting price of 2/3rds the BIN it ran for a week with no bids, but it's still be re-listed. Realistically worth about £450 perhaps..?
There are numerous others on there hanging around at stratospheric prices too, but the owners don't seem in a hurry to shift them.