In the future, every smart contract developer is also a security researcher.

The attack surface is too large for people to have security be considered only during the audit.

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You’ll need to be security minded right when you write the contracts. It doesn’t matter how good you are. If you write smart contracts without security in mind, you’ll write bad contracts

Yes and no.
Its definitely important to be aware of both aspects, but if you're mainly a smart contract dev, then the odds of you also being an effective security researcher are slim. 
It's also good to have others who aren't as as close to your code validate it, as they wont be as prone to certain blind spots

Sorry but no

Specialization wins every time