They are still very niche devices, though. Onyx made a similar thing back in 2013, but they never followed up because it didn't sell well.
FWIW, I do have a Kingrow K1, and I like it for what it is, which is a 5" Kindle that can also run the occasional Android app (like, say, OsmAnd or Kiwix). It's an excellent off-the-grid device due to battery life, which is precisely what I got it for. But for business use, I don't see much point - a dedicated business device to me would be something like Cosmo Communicator or Unihertz Titan.
With K1, I have a 400 Gb card in there that's loaded with stuff - not only my entire Kindle library, but also Kiwix loaded with offline Wikipedia and a bunch of other wikis, OsmAnd topo maps etc. Between that, and several days of battery life with occasional use, it's perfect for multi-day hikes out in the boonies.
If you mean the screen refresh rate, it's typical eInk, comparable to Kindle. For tasks that require fast refresh, it's certainly not a pleasant device to use - e.g. scrolling around in maps is a bit tricky - but it's manageable.
I would pay any reasonable - and even a slightly unreasonable - price for a full E-ink phone, because I am a business user and do not care about consuming media. Likewise for a laptop or desktop screen. Yet they are all but unavailable!