I think it's the opposite. Few people will buy an iPhone 11 now, unless they have to get a new iPhone asap or if they don't bother to google whether it's a good idea to buy one now. Everyone else will wait for the iPhone 12.Think carefully about the consequences of that. Paper launch an iPhone but have no supplies until Oct/Nov.
What do you think happens to the price of iPhone 11? Why would Apple volunteer to Osborne their entire iPhone 11 lineup several weeks before it is necessary and lose $100 per iPhone?
However, if they already announce the iPhone 12 now, release it in October/November, but now already drop prices for the 11, they will sell a lot of 11s again in the next few weeks. Because for many people the extra 100$ for the iPhone 12 won't be worth it. This might actually allow them to see a nice surge of selling 11s now, which is also good because iPhone 12 supply will be difficult in the first few weeks, et cetera.
At least that's how I would expect it to work. I might be wrong of course. I also wonder if the iPhone XR, 11 and SE will continue to have EarPods and a charger. They also removed the Lightning to headphone jack adapter from the older iPhones at a certain point, so there is a precedent of Apple doing that again.