Never said - or even thought - that.
I used the words "base of the carpet fibers" meaning the area at the primary backing. I certainly don't expect any consumer level vacuum to suck dirt from below the primary or secondary backing (or unitary backing, if that is how the carpet was constructed).
I'm not even expecting a consumer vacuum to get *all* of the dirt out, just a lot more than any robot vacuum. The more that can be removed, the less abrasion of the fibers that will occur. I'm as concerned about my carpet looking good, not just clean, for as long as possible.
Said the guy that doesn't have any carpets. ;-)
I would imagine that it doesn't take long for dirt and sand to get to the base of the fibers. Gravity sucks, you know. While a daily maintenance run of the robot will certainly help, I'll wager that you'd be hard pressed to find a robot vacuum listed as a "recommended vacuum" by any of the major carpet manufacturers - some of which even hint at a daily vacuuming in certain situations.
In fact, the Carpet and Rug Institute has certified only one robotic vacuum (out of 314 certifications) and it's not going to work in the average living room.
No doubt.
My thoughts haven't changed:
When it come to carpets, robotic vacuums have their place as an appearance maintainer but not as an overall replacement for a quality upright or power-brush canister/whole-house system. Not if you want your carpet to last as long as possible.