I'm not convinced that this is a widespread problem. With estimated annual sales of 685 million dollars, Specialized sells a lot of bikes, and I've only heard of a very small number of issues of this nature (granted, one of which is my bike). I will not be doing any Loctite for two reasons. First, when I pay a premium, it is partially to avoid having to monkey around. I will drop the bike off for it's recommended inspections, and pay attention to any unusual sounds, and that is it. I think opening it up and applying Loctite could very well void the warranty. Second, I am not worried this will happen again, but if it did, I have even more confidence in Specialized (and my LBS) standing behind the product, given the way they treated me. I personally do not mind getting a brand new, updated bike for my bike that had quite a few kilometres and battery cycles.I am not convinced that Specialized has solved the problem... the frame appears to use the mid-drive motor as a structural element.
If the mounting bolts come loose, the frame could experience additional stress that may cause crack propagation and potential failure.
I see no evidence of frame re-design in the 2020 models... if I owned a Specialized I would use Loctite on the bolts and check the frame regularly. YMMV