A similar method is to put the external call at the very end of your function, after any state updates, and make sure that function isn't called from another function. These are well-known techniques in the community and commonly used, which is probably one reason the researchers didn't find any vulnerabilities in deployed contracts.

The gas limit on send was more of a belt-and-suspenders thing, not intended as the sole protection; that may have been a bad idea in retrospect, but at the time people were pretty focused on adding every protection possible against another DAO situation.

Because the kind of developer the average ICO scam hires isn't quality enough to code this kind of security.