a) what constitutes anticompetitive activity or otherwise warrants antitrust action is likely to always be at least somewhat subjective, and require human judgement, meaning that motivated humans can choose not to do it in many cases, and
b) depending on the degree to which the government is captured by a party that actively works against antitrust laws and believes that bigger[0] is always better, and on how long said capture lasts, it doesn't matter what laws we put in place now. Laws can be changed by the government.
We're not going to be able to make a foolproof system just by making better laws on this one issue. If we believe strongly in antitrust with teeth, we also need to be committed to supporting politicians who favor it, and political and governmental systems that prevent a vocal minority from seizing control of the engines of power, consistently for the rest of our lives.
[0] Bigger payouts for their friends, that is, which means bigger kickbacks now and bigger paychecks later when they go to work for said friends
But regulators stopped enforcing those regulations in the 1980s because of "efficiency" and each administration after Reagan doubled down. Luckily some politicians are starting to see the error of their ways and listening to thinkers like Matt Stoller:
https://mattstoller.substack.com/