I think the bigger issue for both AMD and NVIDIA moving forward is not whether they can design high end cards, aka big dies, but whether the process has good enough yields for such large dies.
Additionally with Samsung 7nm yields not being economical outside of small Smartphone SoCs, then there is the capacity problem with NVIDIA also trying to move to TSMC for their high end at least.
Navi10 is only 251mm2 for a reason not because AMD cannot design a 2080Ti competitor but because in 2019 there not enough wafers to go around (at least for AMD) and the process is not mature enough to make 450+ mm2 dies with a high enough yield. Lisa confirmed of tight wafer availability, which in part is a fault of AMD for not having ordered enough in advance.
Since the chip has a conservative amount of CUs they needed to clock it high to reach the performance target but also losing some efficiency in the process.
I don't believe AMD could not have designed and launched a 2080Ti competitor last Computex if it were not for process/wafer constraints. <450 mm2 Navi10 could possibly reach 2080Ti performance under 300W if they could clock it modestly.
With AMD's console chip also fabbing in H2 2020, there is gonna be a real dearth of Wafers at TSMC. Add to the fact that the consoles are going to be around the 400mm2 die area.
But on the other hand, I have been seeing reports that the yields for bigger dies is improving a lot for 7+. It is presumably cheaper, higher yield and improved characteristics.
There is a reason nobody in HPC is talking about TSMC's 5nm except Smartphone guys like Apple. Ian mentioned some weeks ago yields even for tiny <100 mm dies are horrid for 5nm but at least it can alleviate the capacity problem for 7nm when Apple moves to 5nm.
Additionally this new interest in doing MCM and mGPU.
I think NVIDIA will fab the smaller GPUs at Samsung, probably AMD as well at some point in the future.
With RAM prices and Wafer costs ballooning in 2020 be prepared to pay top dollars for your next NVIDIA/AMD high end. There is a planned increase of WPM in H2 2020 at TSMC but remains to be seen if those are not going to be gobbled up by MS and Sony Orders.
For AMD they would rather use those limited wafers to produce EPYC rather than make some 2080Ti competitor.