Supposing that sincerity has declined, why?
It feels natural to me that sincere enthusiasms should be rare relative to criticism and half-heartedness. But I would have thought this was born of fairly basic features of the situation, and so wouldn’t change over time.
It seems clearly easier and less socially risky to be critical of things, or non-committal, than to stand for a positive vision. It is easier to produce a valid criticism than an idea immune to valid criticism (and easier again to say, ‘this is very simplistic - the situation is subtle’). And if an idea is criticized, the critic gets to seem sophisticated, while the holder of the idea gets to seem naïve. A criticism is smaller than a positive vision, so a critic is usually not staking their reputation on their criticism as much, or claiming that it is good, in the way that the enthusiast is.
But there are also rewards for positive visions and for sincere enthusiasm that aren’t had by critics and routine doubters. So for things to change over time, you really just need the scale of these incentives to change, whether in a basic way or because the situation is changing.
One way this could have happened is that the internet (or even earlier change in the information economy) somehow changed the ecology of enthusiasts and doubters, pushing the incentives away from enthusiasm. e.g. The ease, convenience and anonymity of criticizing and doubting on the internet puts a given positive vision in contact with many more critics, making it basically impossible for an idea to emerge not substantially marred by doubt and teeming with uncertainties and summarizable as ‘maybe X, but I don’t know, it’s complicated’. This makes presenting positive visions less appealing, reducing the population of positive vision havers, and making them either less confident or more the kinds of people whose confidence isn’t affected by the volume of doubt other people might have about what they are saying. Which all make them even easier targets for criticism, and make confident enthusiasm for an idea increasingly correlated with being some kind of arrogant fool. Which decreases the basic respect offered by society for someone seeming to have a positive vision.
This is a very speculative story, but something like these kinds of dynamics seems plausible.
These thoughts were inspired by a conversation I had with Nick Beckstead.