Attacks are still possible, but if we look at xz backdoor attack[1] it was insanely complicated attack and it still failed. Its fail doesn't look promising, attack could succeed just the attacker was unlucky. Still it shows that the success is not guaranteed.
Theoretically npm can be improved in this way, if there were a separate "distro" for packaged, with dedicated maintainers for packages, who don't write code, just pull it from a mainstream and review it. It is not being done because of tragedy of commons, not because it is impossible.