I, on the other hand, am personally hoping the Mk8.5 will NOT push down prices of the 7.5 for obvious reasons! 
Reassuring to hear they remain stubbornly high 
I’m joking of course, we will all have different circumstances and plans. My plan is to keep mine for at least another two years.
yes, get that! but actually think there is a difference between buying and selling privately.
I am really referring to VW Approved, where the prices remain quite high, £25k + for the best low mileage MK 7.5 R and GTI's but you do get the warranty, which for me is worth the extra outlay. I would like that to get down to the £22K mark in about 12 months.
Bought wife a MK 7.5 Golf R last September, well specced, low mileage, for just short of £25k VW Approved (Did actually get an above average figure on our exchange, so our cost for change made it a better deal overall than anywhere elsewhere)
I doubt could get more than £20k privately now, and likely £18-19K in exchange.
Think the private sale value will not be overly affected as its always more attractive than forecourt, if willing to take the risks, but imagine trade in and selling via Motorway and WBAC would take a hit if the MK 8.5 brought more MK 7.5 to market as exchanges (those who have held on the MK 8 and are more ready now)
But if you want to buy a decent, relatively low mileage Golf R or GTI on VW approved, you will still see an asking price we paid in September, and if you look on Autotrader, you can get early MK 8 R and Clubsport for same prices as late MK 7.5
Personally think its testament to how good a car the MK 7.5 is, I missed my GTI TCR so much, that went and bought that MK 7.5 Golf R just to have one back in the family again while see the company lease out on this BMW 128Ti, and when that ends in 15 months, will be getting a MK 7.5 of my own, as cannot trust getting a MK 8 lemon, and MK 8.5 will be out of my price band for at least 3 years.
Reason state out of my price band, is turning my back on lease and finance, its way too expensive now, now market has normalised, there too much depreciation on finance, and so it will be 3-5 year used for cash from herein for me.
Do buy cars expecting to lose money, but not prepared to take the biggest part of that curve from new.
Definitely see me in a MK 8.5 in 3-4 years time, probably a fully specced R or Clubsport to see me through next 3-4 years.