The default tonemapper should be as physically accurate as possible as to provide a neutral starting point. The problem is that the bloom generated by the old tonemapper resulted in a visual effect that, in real life, would be the result of a combination of different phenomena, not only camera lens light bleeding (which bloom is). Specifically, exaggerated bloom gives the impression of atmospheric light scattering (the “hazy” look) and the reflection of diffuse emissive lighting on nearby surfaces. When the tonemapper is made physically correct, those side effects disappear and you only get actual bloom.
Now, UE4 always supported overriding the tonemapper with a custom one in the post processing settings, so even if Epic removes the old tonemapper, you can continue to use it. I think this is a better solution than having everyone design assets around a non-standard tonemapper that doesn’t match industry standards and bias games toward a proprietary look by default.