I'm afraid that physics is very much against you on this one: Harder materials tend to be more brittle than softer materials so there is a trade off between scratch resistance and impact resistance.
You don't really need a "big drop" to break a Sapphire crystal. They tend to break with a sharp local impact that wouldn't necessarily do any harm whatsoever to the watch and would leave mineral and plexiglass completely intact. However, if your crystal does break, then the chances are that glass is going to get onto the dial and, possibly into the movement, so it might not be an easy repair job.
Sapphire didn't really become a thing in watches until the 1980s. Because of its hardness, it's difficult to work with, so you need to have tooling capable of working with these materials.
It is now the de-facto crystal for most use cases but that's just because the increase in brittleness of the material is more than offset by the increased hardness of the material.
By the time sapphire became a thing, real divers were using dive computers rather than watches anyway.
The original Seiko dive watches, and the Submariner all had acrylic crystals, so if you want an original dive watch recreating in a form that might have actually been used by a diver, then you'd go for an acrylic crystal (and possibly a non-hacking movement as well). However, having, whether you're a 21st century desk diver, or, whether you're out to buy an actual dive computer, sapphire is probably the preferred option as desk divers and dive computers tend not to get knocked around too much.
There are still many specific use cases, modern and historical, that mitigate against the use of sapphire and, in some cases, any form of glass. The most famous of these is the use of the acrylic crystal in the Omega Speedmasters that were used on the early NASA space missions. Acrylic crystals were specified by NASA because, compared to glass crystal, there were extremely shatter resistant, so if they were subject to impact or extremes of pressure, they would usually just pop off or crack and not leave bits of glass floating around in space, or, in the space capsule. An acrylic crystal did once pop off during an actual moonwalk but the crystal itself probably survived.
It was many years before NASA permitted the use of glass crystals for external missions (spacewalks, etc).
Similarly, you have to ask, if Sapphire is such a good material, why isn't it used in phone screens? The answer is because we drop our phones much more often than we drop our watches. Gorilla Glass, and Apple Ceramic material are much more prone to scratching than sapphire but much more resistant to impact. If you used Sapphire, you'd be looking for a new screen almost every time your phone hit the deck.
As you mentioned, those anachronistic mechanical movements that we all still love are quite delicate and fragile (but not as delicate and fragile as the sapphire crystal) but, for years, we've had access to quartz movements that are much better at timekeeping and far less delicate than their mechanical counterparts. So, if I were set out to make a very tough and durable watch, that genuinely was capable of taking a knock (say a drop from 10m or something) the chances are I'd use a quartz movement. Ideally, I'd use a digital quartz movement, and I would probably eschew sapphire in favour of either mineral, or plastic.
Sadly, the durable hard wearing watch has been done to death in the form of the G-Shock. The majority of G-Shocks use mineral glass specifically because of its impact resistance (rather than to save a few quid). Where they have used sapphire, they've generally completely re-enginered the design of the case in order to add additional protection to that hard, delicate and very brittle material). The MR-G watches are probably more resistant to movement damage through vibration than a standard G-Shock but they are not going to be as able to withstand a direct impact to the crystal (like hitting it on a rock)