MSAA doesn't do anything for shader aliasing, it only handles triangle aliasing, and modern renderers have plenty of shader aliasing so there isn't much reason to use MSAA.

The high triangle count of modern renders might in some cases cause the MSAA to become closer to SSAA in terms of cost and memory usage, all for a rather small AA count relative to a temporal method.

Temporal AA can handle everything, and is relatively cheap, so it has replaced all the other approaches. I haven't used Unreal TAA, does Unreal not support the various vendor AI driven TAA's?