The problem is that its not a black or white thing. As with most things in life, it resides in they grey zone. Its probably true in most instances, but there's definitely scenarios where its not true. Therefore being rootbound does exist.
"Leaving the plant in that same small container and increasing the amount of nutrients that you feed it @ that time should be perfectly fine for that plant to be ok"
Thats way too broad of a statement to be true or false really. Its a bit of a nothingburger.
"Roots do not need to keep growing the whole time a plant is alive in order for it to live happily and grow. The only thing that is needed for a plant to live happily is all its conditions being met".
Yep, thats mostly true. But it is kind of besides the point...so its another nothing point.
How small of a container? 500mL? 5L? 20L? It matters. I agree, as per your second point above, that a plant will live happily even if its roots arent expanding. So, going by that theory, I can easily grow a 3m tall monster in a 500mL pot then. Right? No, obviously. Yes. It'll LIVE happily in a 500mL pot, but will it GROW...to 3m tall? No. Because it'll be root bound.
I mean, the roots can only absorb so much nutrients at a given maximum rate. The roots ability to absorb isn't infinite. There's obviously going to be a point where the required nutrient uptake to sustain the above ground size overtakes the roots ability to absorb nutrients given a restricted root area...and the plant will stop growing...it will merely sustain. Its still living happily, but it isnt growing any more. The size of the container will determine what that level is.
Just saying "small container" is pointless. And if you then say something like "well, you know what I mean...something reasonable sized like what most people use", well, thats irrelevant and actually disproves the original point you were trying to make...which was a sweeping black and white statement saying being rootbound doesnt exist. The condition doesn't exist only if you exclude the scenarios where it does exist.
And just increasing the nutrient load isnt necessarily possible either. Nutrients are chemicals, whether natural or artificial, so there must also be a point where you csnt go above a certain strength or else it will "burn" the delicate roots. I dont know what that level is, but I feel like a rootball the size of my fist cant support a 3m tall monster.
Not to mention the fact that a 3m tall plant in a 500mL pot would need to be watered almost constantly. So there's that to consider too. If you can only be there to water the plant at certain intervals, like in the morning before or afternoon after work, then a small pot may dry out too quickly. Again, it'll live, but its growth is going to be impacted by the dry spells where its not getting the water that it wants. Even if you dont go with such an extreme example of a 3m plant in a 500mL pot, a plant could essentially become "situationally rootbound", if you see what I'm getting at...?
Like, if the container is big enough to supply the required nutrients, but not big enough to store enough water to get by between waterings then the plant will be stunted. Plus water helps the roots access nutrients. Its not rootbound in the traditional sense but I would still classify that as being rootbound. If it was potted up, the accessible water reserves for the roots would be increased even if the roots dont expand, which would thus allow adequate water reserves to get through dry spells, and being moist for longer would increase access to the adequate supply of nutrients in the soil. Result: Increasing the pot size has helped the plant and allowed it to resume growing, and in a round about way has incresed the access to nutrients even if the roots havent expanded.
I mean, just look at the whole art of bonsai. You can keep large trees small by limiting root space, and periodically trimming the roots. Yes, they do other things like training and pruning. But they are more after the fact things and contribute less to the size of the plant and more to the form. The massively restricted roots are what keeps them small. "Keeps" being the word to note. They are happily living, yes, but they keep at a small size...they dont grow much, to the point that minimal pruning can keep a tree at 2ft that should be 150ft tall.