I'm a bit of a matter of fact type of person, I observe first, then see about testing some idea. Then see what I might be able to conclude.
EI is simple, it's suppose to be, it's the most reduced type of dosing method.
Water change: removes any excess build up, dosing, frequently: prevents anything from running out.
Some math can be used to estimate what % error you have, for 50% weekly water changes, 2x the dose for the week.
Teaspoons are accurate for our purposes and easy to use.
DIY chemicals are very cheap and also standard, so anyone anywhere uses the same stuff.
This is simple. There's not a whole lot to it. Folks ask a dozen questions about it still, no matter how simple I explain it.
They worry something will go wrong, that it will cause algae, harm their fish or any several dozen myths that have been said and are still being claimed even after 15 years of it's use and a decade of wide spread use.
LFS's and vendors do not like EI and other DIY methods, they cannot make $ off them like ADA, and other brands.
So they will scare and warn folks about such methods and further the myth simply to make some sales...................
There is no other reason other than ignorance, I'm not sure which is worse.
=>>>> In the reasoning you state, EI simply rules out nutrients as dependent factors. So the other dependent factors are light and CO2. Now we only have 2 things to deal with instead of three.
Light is fairly stable and if you had been able to grow things in less than optimal nutrients, now you should be even better off.
Now you are down to one thing: CO2.
You can reduce light also, this will reduce CO2 demand and also thereby.......nutrient demand.
This will make targeting a good non limiting CO2 level fairly easy.
This gives you an easy to manage system where no testing is needed, low occurrence of algae, good healthy growth with good color.
It's just using common sense to isolate things to one main variable, CO2.
CO2 is the trickiest of the parameters to measure, the largest % of plant biomass and the most limiting factor for growth, the one that can change the most in a few minutes/hours etc.
If you want to induce algae, try turning off the CO2 for awhile and see how long it takes to get bad algae.
Nothing does this like poor CO2..............
If you cannot knowingly rule out CO2 and test the other parameters, you are in a lot of trouble experimentally. You have no confirmation and I do not trust the pH/KH test one bit, nor Drop checkers etc. I've measured CO2 very critically and it varies more than most aquarist realize.
The issue is really about EI dosing here, it adds a nice non limiting concentration to plants. That's all it does.
A few simple test falisfy these other myths/claims against it and are and have been repeated many times for over a decade.
This is not anything "new".