I agree with you and think that Shimano (and others) will keep supplying some 9-speed parts for a number of years, but those new 9-speed parts aren't going to be high-end ones. If you need replacements for old Ultegra 9-speed parts, for example, you'll need to find them used or new-old-stock. If you replace with new parts, they'll be from downlevel groups like Sora. (Not that Sora stuff doesn't work, but the fit & finish arguably aren't as nice.)