I just do not see that recursive self-improvement will happen any time soon and cause an intelligence explosion. What evidence is there against a gradual development?

Darwinian gradualism doesn't forbid evolution taking place rapidly. I can see evolutionary progress accelerating over the course of my own lifespan - which is pretty incredible considering that evolution usually happens on a scale of millions of years. More humans in parallel can do more science and engineering. The better their living standard, the more they can do. Then there are the machines...

Maybe some of the pressures causing the speed-up will slack off - but if they don't then humanity may well face a bare-knuckle ride into inner-space - and fairly soon.