Because markets are based on competition, and that causes a lot of issues on it's own, even in the absence of capitalism. It's because of competition that we consume too much resources, have wars between nations, etc. I don't think we can get rid of a lot of the problems we have unless we start actually planning long term and co-operating instead of competing. I agree with you that it'd be hard and risky to build a working planned economy, and that it'd be best to start with small scale tests. I just don't see how humanity will survive unless we start planning at a very large scale and stop competing with each other. A global planned economy is the only way to do that.
> But I believe most people, including myself, would only do the minimum if no personal gain was involved
I don't think all personal gain needs to be removed in a planned economy. There could be different rewards for different tasks, if achieving the plan requires it. What is not necessary is the ridiculously large differences between rewards. It's enough that the reward is somewhat motivating, there's no need to have some tasks be thousands or millions of times more rewarding than others. If there are tasks that wouldn't get done in the absence of rewards so large, surely they are rare and worth leaving undone for the other benefits of a planned economy.
> there are a number of social democracies (which are mixed economy, i.e. capitalist) in Europe that attest to that.
I live in one of the best managed ones, and still we're slowly but surely losing our government to the influence of capital. Media is concentrating, worker rights and unions are being attacked, etc. It takes a long time in a well designed system, but eventually capital will win. Even if we manage to get a government that tries to protect the rights of people over capital, the realities of a global economy based on competition don't allow that anymore. If we try, we lose to countries that don't and then we can no longer afford it.