(1) From a dialectic point of view, I think Kurwzeil makes an impressively compelling argument that the rate of progress has been exponential, historically (e.g. using others' milestones). So it's striking that he doesn't argue for an underlying mechanism, nor for whether it can continue - or even mention it explicitly. The mechanism seems to be similar to standing on the shoulders of giants - once an improved techno…
Consider you have a container of numbers, starting with a single number inside, 1. Each turn, you can add any number of pairs of two numbers inside it (with replacement, so you can add two 1's), to discover more numbers.
The first turn you can add two 1's to discover 2. The next turn (with 1, 2) you can discover 3, 4, the turn after that (1, 2, 3, 4) you can discover (5, 6, 7, 8). The number of numbers you can discover in any single turn will continue to increase as you increase your number of discoveries.
I am thinking the density increases because with more discoveries you can form more permutations of existing knowledge.