there is a bit more to it, but you are also not wrong, first thing is to do colormanagement at all, most just throw in a linear/AlexaWideGamut plate and render linear/709 CG on top and then use color correction in comp to put it in the right place, goes all the way back to how textures are painted and how lookdev is beign done, things can be insanely skewed.
If you want to use linear/709 as your working space thats valid, and you can still use the great aces Tonemapping view-transforms to your advantage.
If you are using EXRs for exchange in linear/709 you are also not loosing much color information if at all, you can expand it back to alexa-wide gamut for grading with almost no loss, only thing where you need to be careful is when you use LOG in between in your comp, negative values , which you will have lots of, can lead to loss(although thats a edge case but can still happen with very saturated things and noise) .
But, there is a bit more to the aces advantage if you look into Lookdev and lighting, there are a bunch of great articles outlining the effects of Global Illumination when using higher gamut textures and it can be amazing for Neon lights and a bunch of other things that come with a extended gamut, even if your textures are still sRGB/709 .
Basically you cant recreate anything outside of 709 in CG when you do this, but your plate can, if you then pull around the image in grading it can fall apart quickly, how would you match a 3D lightsaber to a real one when its allready out of 709 gamut when filmed? Can you fix it in comp, yes you can if you are aware on how to handle negative numbers…
Basically proper lighting and texturing in a gamut close to your plate gamut will make compositing a lot easier.
Added to that , its been some great development happening for lighting that makes the whole process a bunch easier, most renderers have some kind of OCIO or aces support, I think even Octane now, I mainly use vray and its implementation was bad but got better with V5, but all in all the only missing pieces are Substance Painter and Photoshop, once those are ready there is really nothing in the way of just doing everything in acesCG.
Also, you probably also have the case of asset reuse-ability, lets say you need to deliver a HDR commercial of a previous 709 show in a few years, you want to just pull out the ACES master , do a quick trim grade and be done .