It would cost more due to the cost of 7nm and them being only able to make a few. Even if they could do 7nm that cheaply no one would buy it - no one was buying the 290 when it was competing with the 970 (18% market share for AMD then), and that was a lot more competitive. Now all AMD has is the same GCN which is limited to 64 rops/4096 cores and doesn't have ray tracing or AI. That's forces it to be mid range at best, so it's no $600 competitor, it's a $300 part at the most. The numbers don't add up.A 7nm card from AMD now will not obsolete anything, it will only be a high-end product selling at 600+ USD leaving the rest of the line intact, much like NVDIA did with RTX cards.
AMD are not stupid - they aren't going to waste money fighting a loosing battle with Nvidia when they could make much more money making CPU's. That's got to stay the focus. The gpu team can make the next console chips - another pretty assured source of income. The PC market is basically Nvidia's for now.