Wouldn't be better a new complete rewritten engine?

[XTF]

But then again, parts of the the WOL, used so far back as RA iirc, interface crop up in RA3, so I guess that RA3 is a modified RA engine then?

What parts?

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You'd only have to look at the ingame screen to notice they're very different - RA uses flat terrain and 2D graphics that don't really go beyond basic SHPs; TS and onwards use a dimetric camera, TMP terrain, voxels, the list goes on.

That's just the presentation layer.