Saying "The rack is Reason's virtual rack, and the mixer is Reason's virtual mixing console" is not clarifying anything. You're just saying a rose is a rose because it's a rose. It's also false: You can mix completely within the Rack and bypass the main Mixer completely. Reason has been able to do this since at least version 3.
Here's the key point: Rack,mixers, sequencers, combinators... These things don't exist. What they are is representational paradigms: symbolic constructs used to help us understand what they do and use them easily. It's the interface: sectioned-off, related toolsets that we're able to deal with easily. It's entirley possible to make a Subtractor without the front-facing plate and just enter values into several web-like form fields and have a synth make some awesome sounds.
It's just impractical to do.
The purpose of the rack, indeed the three general work areas of Reason, is to compartmentalise the way we think about making music. Create and stylise here (Rack). Arrange there (Sequencer). Unite here (Mixer).
Now... Let's address the look of the Mixer with swappable, modular EQs.
Let's say the 'flavour' I want is a 32-band EQ, because the sound I'm mixing is a live recording that suffers from complex interference waves from an untreated environment. The challenge would be to build that EQ that not only fits into the Modular EQ space, but is still easy to use, and has some frequency shift options to cater for the mic shifting location in the performance space. Now, the most logical practical answer here would be "Dude: that stuff isn't done at the desk-level: you need that in the rack", and I'd agree personally. But the point is, it's a 'flavour' of EQ I want, and it will need more space than the current on-board EQ.
Let's go the other way. What I want is a new thing called a sidechain EQ. It uses the frequency responses of one instrument to determine the EQ of another. It's just a 3-band EQ, but it has a couple of other buttons for tailoring purposes. Where will these buttons fit in the current EQ strip?
I think what I'm missing here is a look on how you think something that has the functionality of Reason's BV512 or a Uhbik Q will still have meaning or be easy to use in the mixer.
Reason's central paradigm is to be easy to make music. This suggestion seems to be about designing audio engineering studios that will require soundies to re-learn Reason for every installation they come across. It also means third party developers would have to cram their unique sound processing tools into two form factors: rack and mixer. Seems a waste.