What's hard to believe is that it would be a far larger jump than we've seen in previous generations of game cards. While it's all well and good to speculate about game card speeds based on what SSDs can do, they're only partially related technologies, and what Nintendo uses is a niche format where there's one major supplier (Macronix) and one major customer (Nintendo), so there are likely very few people outside those two companies who know what is technologically or economically feasible. What we can do, though, is look at the historical trend and try to project from that.
There have been three generations of Nintendo hardware using the same or similar game card technology up to this point, which have the following read speeds:
Nintendo DS - 4.2MB/s / 6.7MB/s
Nintendo 3DS - 16.6MB/s
Nintendo Switch - 50MB/s
That's around a 3x improvement in speed each generation. Absent of any other information about the underlying technology, I don't think it's unreasonable to expect the trend to continue with about a 3x improvement with [redacted] game cards.
For comparison, let's look at the progression of internal storage speeds. The DS didn't launch with internal storage, but the 3DS had 2GB of flash memory built in. I haven't been able to find reliable information of the speed of the 3DS flash, but we can make a reasonable estimate based on contemporary devices. The Samsung Galaxy S2 launched at about the same time with 16GB of storage, and I can find
a benchmark showing peak read speeds of 47MB/s. I don't think the 3DS would have been using storage quite as fast as a flagship phone, so let's say the 3DS's storage was somewhere around 30MB/s. The precise number doesn't matter that much.
As confirmed by users with hacked consoles, the original Switch's eMMC is capable of 300MB/s, even if games weren't able to utilise it due to CPU bottlenecks. So 200MB/s storage in [redacted] would be a downgrade over the previous generation, which would be an interesting choice to say the least.
Anyway, for internal storage, we can compare two of the consoles we have above:
Nintendo 3DS - ~30MB/s
Nintendo Switch - 300MB/s
That's a 10x generation-on-generation improvement. I don't personally expect another 10x jump here, but the rate of improvement in the technology available to Nintendo hasn't slowed down. At the time the 3DS launched, the fastest phone storage was around 47MB/s. At the time the Switch launched, it had
jumped to 485MB/s. The fastest phone now has storage
capable of 4.5GB/s read speeds. If we're assuming that Nintendo was about 33% behind the state of the art with the 3DS storage, and was about 33% behind the state of the art with the Switch storage, then 33% behind the state of the art now would be 3GB/s, another 10x jump.
When we've got one format with an exponential growth rate of 3x per generation, and another with an exponential growth rate of 10x per generation, the gap between them also grows exponentially. The 3DS internal storage was (at a guess) about 2x as fast as the game cards. The Switch's internal storage in 6x as fast as the game cards. Projecting out, we would expect a next generation console to have internal storage 18x as fast as its game cards. This kind of mismatch is why mandatory installs became a thing for Sony and MS 10 years ago.
The 3DS only had a small difference between internal storage and game cards, and although the Switch had a larger one, CPU limitations meant the real-world performance difference was similarly small. The existence of the FDE means there shouldn't be a CPU bottleneck on [redacted], though, so it's entirely possible that we could be looking at a real-world performance difference of 10x between internal storage and game cards. That's not just a "better experience", that's games built around internal storage just straight-up not being possible on game cards.
I also disagree with the notion that mandatory installs "won't go down well with anyone". Mandatory installs have been standard in the rest of the industry for a decade now. There are kids playing games who literally haven't lived in a world where you can play a game directly off physical media on anything but a Nintendo device. Nintendo's the exception here, not the norm. Besides, if game card speeds progress as we'd expect them to, we'd be looking at 32GB games installing in under 5 minutes. That's a minor inconvenience at worst. I fail to see how forcing ambitious games to be download only is a better solution than that.