What if (in a real situation) you lose the ability to get air from the tank AFTER you have exhaled? What if you were deliberately heavy when the problem developed, like say you were on the bottom and trying to be stable to take a photograph? What if there was no way to add air to your BC? What if you (as some people advocate) have zero ditchable ballast?
I can think of a lot of potential situations, which COULD make an actual CESA much more challenging than a relaxed 30 second swim in a pool where you know the surface is just a few feet above your head
I can hopefully answer this and put to bed other people's "Pet theories" since I've actually made a CESA for real
In my case I hadn't run out of air, I had two redundant cylinders, each over 3/4 full - with was rather annoying - I had a Laryngospasm
The symptoms are the same, you can't breath - although in my case I was looking at 2 SPG's mentally saying WTF!
I happened to be a 22m in crystal clear vis of the Red sea - so the surface seemed within touching distance. and at that point had 500+ dives behind me in fairly challenging conditions
Of course you only realise you cannot breathe after the exhale (unlike in training where you prepare yourself)
I did waste a small amount of time by swapping regs (spitting one out putting the other in - automated response) I probably took 10 seconds in all before I fully reacted and headed straight up.
According to my computer my ascent rate was around 20m/min - I don't recall it screaming away - I had more pressing issues. I do (still) remember the clarity of thought - the best way to describe would be for the reader to imagine reading 4 different columns of text, simultaneously and understanding them completely
Your diaphragm is working hard trying to inflate yoru lungs (they can't ) if I experienced any expansion of the gas in my lungs, I certainly didn't feel it.
My focus was on the surface. I do remember thinking it seemed a long way away. I was kicking hard (using dive rite XT's) so using oxygen, again I remember it felt like hard work.
In the end the Laryngospasm clear at approx 10m, my exhale was so violent that my mask flooded. That was when I saw my buddy. She'd seen me fly off, at first thought I'd seen something big, but then her instinct took over - she told me later she's had trouble keeping up with me.
Post dive, in hindsight, I realised I might have done things differently (I wasn't releasing air from my BCD) -
Had I thought about all my training, I'd have ignored it anyway, since my priority was to be on the surface. I suspect most people will do the same. I wasn't concerned with a controlled ascent rate. Not being able to breathe meant I wanted to be at the surface its as simple as that.