This is simply not true - it assumes that everyone's forwarding server for an unknown record is the root servers directly, which isn't the case.

If this were true, my life would be much easier because DNS propagation would be much more predictable than it is today. One bad server in the chain serving obnoxiously high and invalid TTLs can ruin your day. It's not very common percentage-wise, but it certainly happens every time we switch DNS over for Stack Overflow.

"because if everyone did it you would bring down the root servers."

What figures are you basing that on? The TTL for the NS records for '.' is 6 days. Or are you just guessing? I think it would be perfectly feasible for everyone to run a local recursive resolver. As long as the change was gradual rather than over night.