It is very unlikely that you'll shatter a sapphire crystal. It's very likely that you'll scratch a glass crystal.
Overall, shattering either a sapphire or glass watch crystal is unlikely. Scratches are a far more common occurrence, which is why people pay more for sapphire crystals. For the most part, it is a matter of appearance, rather than resistance to breakage.
Of course, scratches are points of weakness that reduce a display's resistance to breakage. One can say a sapphire-covered display will better maintain its resistance to breakage over time, while a glass-covered display's break resistance is more likely to degrade over time.
All this "brittle" stuff is besides the point. Everything reaches a breaking point. What matters is when that breaking point is reached (how much force is required). A brittle substance may break with very little force, or may require a whole lot of force. Sapphire happens to require a lot of force. Peanut brittle (candy) requires very little force.
I had the sapphire Watch (first gen) and now have the glass Series 4. No scratches on the first gen after years of use (well, the stainless steel scratched). No visible scratches on the new one either, but it's only been a few months. Over all the decades I've worn watches, I've never broken a crystal. However, I've managed to scratch them all (some quite badly), with the exception of that sapphire Watch (the jury is still out as far as the new Watch). Again, the primary value of sapphire to me wasn't the risk of breakage, but the scratch-free appearance.