We ultimately don't know what the perfect amount is (especially since we don't have Navi 22 or 23 products to test yet), but considering that some testing by users who are overclocking the memory show good performance gains from doing so, I think it's somewhat reasonably to conclude that Navi 21 probably doesn't have enough bandwidth or that it could use additional infinity cache such that the hit rate is high enough to better compensate for that. Some of the comparisons against the latest Nvidia cards which do have much higher memory bandwidth show that AMD falls off a bit at 4K relative to the other resolutions and the lack of memory bandwidth could be a part of that result.
If you look at the data AMD presented, it's pretty clear that at 128 MB, 1080p games have pretty much hit the point of demising returns at the curve for the hit rate will be essentially flat. For 4K gaming, it looks more linear up to and including 128 MB, so unless it falls off just past that (which seems unlikely) additional infinity cache would improve results. I expect that AMD will include more infinity cache when they move to 5nm. Infinity cache works better at a conceptual level the better the hit rate is and does a better job at compensating for a lack of memory bandwidth. Even being able to go from a hit rate of 70% to 80% is massive. It might only seem like a 10% improvement when viewed that way, but it actually represents a 33% reduction in the times you need to go out to memory.
Of course this leaves a question as to why AMD wouldn't include as much as they perhaps should have with Navi 21, but I think that it's somewhat obvious why given that Navi 21 is already quite large. It's the second largest die they've ever released as a consumer product with only Fiji coming in larger. They may have also anticipated better results than they were able to actually achieve, so it's possible AMD undershot. Another possibility is that AMD was able to get far better clock speeds than they may have initially anticipated, which would also lead to an imbalance.
There's also the nature of graphics themselves. If I want to use a set of textures and other data to construct a 1080p image, I can do so in some fixed amount of time. If I want to use that same set of data to construct a 4K image, I have four times as many pixels to push which will require more shaders to do that work in the same amount of time, but I haven't necessarily increased the amount of memory bandwidth I need by very much. Of course most games will use higher resolution textures, etc. when displaying at 4K as opposed to some other resolution, but the amount of data needed and the memory bandwidth required to feed the additional shaders isn't going to increase by four times. So I don't think there's a linear relationship between how many shaders (or other hardware resources) a GPU needs and the amount of memory bandwidth it requires to keep them all fed.