I find the Leica digital holding their value thing to be a myth to a certain extent.
Just look at how the math shakes out:
B&H has the Nikon D850 selling for $2398 new, and roughly $1600 used. 66% of it's new value.
The Leica M10 was $6595 when it first came out, now it's selling at KEH for $4563, so 69% of its new value.
Roughly the same level of depreciation.
The M240 is trading at roughly 50% of the new price. The same year the M240 came out, the D800 was released. This camera is significantly cheaper, at around $500.
For the older cameras you're talking about the M9 or M8, and you really need to be an aficionado to appreciate those cameras...they're absolutely not for everyone so it doesn't make a ton of sense to include them comparatively. Personally I would argue that relative to their 'actual usefulness' and 'reliability', they are unreasonably priced. They're more likely at this point to brick themselves than keep working for years. Even the memory card compatibility is becoming problematic. For a camera of a similar era, the D700, you have a much lower barrier to entry. You can get one under $400 which is completely appropriate. And the same 'stabilized' effect applies here. The D700 is probably not going to drop much further than that. However, the M10 and M240 will continue to drop in price as new models come out, just as the D850 will too. Both of them will eventually reach a 'floor' of value...and by that time only certain dedicated people will be seeking them out.
TL/DR, these cameras all hold their value in roughly the same way. It's just that Leica's require more up front investment. And you should also question whether it's 'good' that you have to pay $3k for a 2012 camera that has a lot of sensor banding at high ISO, even if someone else will pay $3k for it when you trade it.
For the film cameras, I think about it differently. You can certainly feel like you're parking money in the value of an M4. You take care of it, and you'll get it back. The digital Ms will eventually just become unusable.
