First of all, OP is a good read, even though I disagree on main points.
The issue of hard limit boils down to psychology and culture. It is stated in the OP how the supply of an ideal currency should scale with the economic growth or recession. In that case, we need to think in terms of changing supply and (ideally) steady real value of the unit of currency. In reality, however, this implies trust in a central authority issuing and destroying currency at their (corrupt) will. It implies uncertainty. Importantly, we have all seen how dysfunctional and corrupt this system is. Taking "experts" seriously would mean mental disease at this point.
Bitcoin, on the other hand, offers a mathematically certain supply (with some minor variation due to hash rate variation and compressed or extended schedule of difficulty adjustments). In the case of Bitcoin, therefore, we need to think in terms of scheduled supply and variable real value of the unit depending on the growth or recession of the economy. If the economy is growing, value of coins is rising, and people may slow down on spending and increase savings, thereby slowing down the economy, thus leading to a slower increase or even to a decrease in the value of a coin, which then stimulates spending, and drives the economic growth... you get the picture. Hard limit on the supply leads to a sane, self-regulating economy, as opposed to the current mainstream idiocy.
Niko, I read this again, and I think I have just fully understood it. So what you are saying is that keeping the quantity a constant while allowing the value to change is just the other side of the same idea, which is allowing the quantity to change, thereby steadying the value. Sort of like an adiabatic pressure, volume, and temperature (velocity) relationship. Keeping the pressure (price) constant and allowing the volume (quantity) to change has a similar effect on temperature as keeping the volume constant and allowing the pressure to change.
You make a very good point, but there is just one thing we need to take into account. If the current Bitcoin rules are followed rigorously in time, there would be a tendency for the absolute quantity to decrease. This is because, like real coins, Bitcoins can be lost. I have a Bitcoin address for which I lost the private key. Fortunately for me, that Bitcoin address has zero coins in it. We can imagine this happening to millions of people on a daily basis losing the private key for one reason or another, and the sum of Bitcoins lost in this manner can be substantial. As another, more dramatic example, some wealthy person with millions of Bitcoins in several accounts or addresses dies, and nobody, not even his heirs, are able to get to the private keys.
One other thing: it's not just economic activity that's going to affect the value of Bitcoins. Things like the size of the Bitcoin economy in terms of total number of users is also a big factor in its valuation. We want to see the Bitcoin market increase in size in terms of number of participants; but even if Bitcoin conguers the whole world this increase in market size will continue due to population increase. At some point (maybe after we have gone to other habitable planets light-years away, and Bitcoin remains the dominant currency), even the fact that each Bitcoin is divisible won't be enough for it to continue to be useful to a population much larger than its quantity.