Tomato plants HAVE THE CAPACITY to root anywhere along the stem, so why don't they fill the gaps in the videos if it's advantageous?I agree with what you say except for the "replacing" part. The bottom roots are not defunct. They still uptake nutrients. What if you laid them on their sides? The new roots are at the same depth as the new roots and when you pull them up the bottom roots have grown just as much as the top roots. That's why it makes no difference between on their side and deep. That issue and the moisture at depth encourages me to plant deep.
Simple. Roots of that type, at that depth are useless, so they only grow at/near the surface again. Hence, the roots you bury deeper are defunct. QED.
"What if you laid them on their sides? The new roots are at the same depth as the new roots and when you pull them up the bottom roots have grown just as much as the top roots."
Nope, the bottom roots, when you plant deeper, do not continue to grow; they stay the same as when you buried them.
"They still uptake nutrients."
Nope, that's the tap root.
" I plant my tomatoes about 2 feet apart because of the intense sunlight. By laying them on their sides I am getting a little too close for the neighboring plants horizontally growing roots."
There's still no advantage to planting them deeper; you're wasting part of a short growing season. If you want the stems to thicken up, get a fan on them.