You are asserting that a free market is unstable and will inevitably lead to a monopoly. Even if I take this as a fact for the sake of argument, it doesn’t nullify my point that judging the intention of political action (antitrust laws to prevent monopolies) against the outcomes of a system (free markets devolve into monopolies) is comparing apples to oranges. You must judge the outcomes of both systems. As I stated in my original reply, regulations can actually lead to monopolies, so the outcomes matter a lot more than their good intentions.
If I don’t take your assertion as a fact though[1], then what you’re doing is judging what you believe to be the outcome of a free market with what you believe will prevent that through legislation. This is entirely too theoretical and doesn’t even begin to answer which is a better system. Ultimately we need to understand the actual trade-offs that are being made between these systems in order to select the system with the most desirable characteristics.
[1] It is not at all proven that a free market will naturally devolve into a monopoly. This has been a contentious debate in economics for centuries and is absolutely not resolved. People tend to assume a static market and extrapolate into the infinite future, e.g. if a horse drawn carriage manufacturer has 99% market share then will it forever be a monopoly in a broken system or will a fledgling automotive industry dethrone this “monopoly” with a better alternative that is not even a direct competitor? This is a really, really deep subject in either case.