First of all, sustained loads over 400W are not very common, if you're looking at the entire population of PC users.When gaming it can go from 300W up to 500+W so then the heat from conversion is 30~50+W. The 40~50W cant be cooled passively without damaging the PCU components in the long run of years.
Second, just because the PSU itself is fanless, it doesn't mean that it's cooled with no airflow. The higher CPU/GPU loads typically result in higher case airflow, which cools the PSU as well, with proper installation.
Third, the fanless PSUs on the market are usually more efficient than 90% at high loads, at least on 230V. I don't see any current models lower than Platinum.
Finally, I'm not saying that fanless PSUs are suitable for every need or every budget, but I do think they're underrated and underused overall in the PC market.
I also believe they're ever so slightly better at passive cooling than fanned PSUs in zero-rpm mode, because they're designed like that from the ground up.
You pay a small price premium and you need some common sense to install one properly. But in turn you get almost certainly less noise and the peace of mind of not having to deal with another fan, which will have to be serviced sooner or later (and it's not as easy as swapping a case fan).