I don't see how a good keyboard case wouldn't work in that scenario.
I also do not agree with your reasons for why they are not produced - your suggestions have been refuted elsewhere in the comments - For examples, the keyboards have been very reliable and rarely fail.
Some of them are great. Others have had hinge problems, the keys have bad tactile feedback or are too close together or have otherwise been panned by reviewers, and other assorted problems.
The reality of manufacturing is that the more components you add, the more likely one of them is to cause problems. This is especially true for moving parts.
Look, I'm not saying you don't have a valid reason to want a modern smartphone with a physical keyboard.
It's just that having been briefly involved with mobile phone manufacturing, I understand the realities of mass-manufacturing complex devices with increasing variability between models to satisfy market requirements.
Unfortunately for you, your market segment is just too small to justify the problems caused by adding a physical keyboard.
Hence my suggestion that an external keyboard might be a good halfway solution. Just like my phone doesn't come with a car holder, so I got one separately for when it's needed.
You could also say that the market isn't really small just nobody is even trying to satisfy it, which may be a valid argument but it seems to me that some manufacturers will try to sell such devices from time to time and it's telling that they don't follow up on those efforts.