This is a great question.

But there are at least two kinds of downhill descents to consider. There are mostly straight smooth pavement descents, and then there are winding technical descents with lots of sharp turns (and sometimes bad pavement).

With straight smooth descents, yes, in most cases, you can get used to using your aerobars going fast downhill. But you better hope that no person or animal or vehicle gets in your way. Because, while you are going very fast, an emergency stop will be very difficult to rapidly accomplish from your aerobars (you have to move to your base bars and then grab your brake levers as fast as possible). But if you are willing to accept the risk, then it is possible to fly down steep smooth hills on your aerobars (with lots of practice of course).

However, on technical descents and/or those on poor pavement, it is another matter altogether.

If the descending road has lots of sharp turns, you need to be on your base bars so you can be on your brakes to modulate your speed as you enter and exit each turn. However, if the road is fairly steep, your bike will be angled down (which will angle your base bars down) and you will have to grip the bars very securely to not slip off the front of the base bar, especially as you add the deceleration forces that try to throw you off the front of your bike with each braking effort.

However, with nearly all TT/tri type flat base bars, when you open your fingers to grab the brake levers, you then lose your grip on the handlebar itself. And if you add bad pavement to the mix, the intense handlebar vibration at speed from the pavement bumps makes it even MORE difficult to hold on to your bike. So if you try to combine a steep downhill, lots of turns, and some bad pavement, if is very very difficult to ride down a steep technical descent at high speed on a tri bike (with a triathlon cockpit). So the compromise is unfortunately to go slow, which reduces the forces that try to throw you off the bars.

So why is it easy to do this kind of steep descent on a bike with drop bars?

The main reason is the shape of the bars. The lower 'hooks' on the handlebars allow you to hold the bar and easily anchor yourself against the downhill slant of the road and the deceleration forces from braking. But, further, the handlebar shape allows you to do this withOUT having to have a death grip on the bars. Actually, you can easily descend and brake with a fairly loose grip on bars and still have your fingers open and wrapped lightly around your brake levers no matter what the forward forces are. Because it is the shape of the bar that holds you, not your grip on the bar. This is what inspires total confidence on high speed technical descents with a bike with drop bars. (It also helps a little that the drops are typically lower than the base bars on most triathlon set ups, so when you grip the drops, you lower your body more than on a tri set-up, which lowers your CG and makes for better bicycle control, especially when cornering.)

Unfortunately, super nice and aero triathlon/TT cockpits are just poorly designed for descending on most real-world roads ...

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