This happens every single release though.
The production of the older gen card gets curtailed and stock levels controlled before a release. Remaining stock at launch of the next gen is sold at discount.
If they release a 1070 that is say 5% slower than a 980Ti and a 180 that is 15% faster the 980Ti can drop in price somewhere between the 2 and sell off the old stock. The 980Ti is a cheap-ish card to make so they could lower prices a lot so they fit within a lineup.
The only card that wont make sense is the Titan X, but that probably has minimal stock now - once the ti was released they knew sales would be low and so have had a year to clean the supply line.
I'm not saying they will kill the 980Ti if it is selling well but at some stage there is enough market penetration that 980ti sales will slow to crawl regardless. And that is ignoring what AMD may ir may not do.
So... assuming "big Pascal" isn't due for some time (2017?), they are going to go from having a line-up which includes a £500 2nd tier card and a £400 3rd tier card, to a new line-up with only a £400 top tier and £300 2nd tier?
And in doing so kill sales of their current £500 GPU that's selling like hot cakes. They'll do this because?
Instead, why wouldn't they launch the "full" mid-sized Pascal for £500 ish, which will beat a 980ti by 15-20% or so, and a cut-down mid-sized Pascal for £400 ish, which will match or slightly beat a 980ti. And/or they could do a £300 2nd/3rd tier card which is roughly on par with a 980 non-ti.
Those prices still represent a £100 saving on today's performance points.
The only way 980ti performance would be lowered to £300 is if they had "big-Pascal" ready to launch imminently. Otherwise, they'd be killing sales of a £500 980ti, and for what? To make less money?
Since Titan nV have a good handle on the amount punters will pay. They aren't going to give anything away for free.