That's such a simplification of the real situation that it's harmful to apply as an axiom.
There are many levels of the marketplace of ideas. Ideally, the gatekeepers of knowledge also create a marketplace of ideas so that expert opinion is varied and shifts as new information come in. This is in inline with what we're seeing here.
The marketplace of ideas with no experts to guide discussion often results in crank ideas that seem plausible but heavily influenced by our biases bubbling to the top. That's how things like the Anti-vax movement gained a foothold.
Also, the "gatekeepers of knowledge" have a mostly.....negative past when you look at the sum of recorded human history. Are you arguing that humans today are just way better and far more trustworthy than the rest of history?
The gatekeepers of knowledge have a past that reflects those that are in power. People that challenge them can be on the right or wrong side of history. Just because they were wrong in the middle ages doesn't mean the gatekeepers of today are wrong. If the gatekeepers can and do apply scientific principles, then logically it is a self-correcting system. This is in theory what we see more or less today (or should at least).
Furthermore, the gatekeeper system is not mutually exclusive to the marketplace of ideas model, as the latter operates on many levels. However, by bringing down the gatekeeper model, it is harder to enforce discussion based on scientific principles, merit and sound arguments. This is the exact reason why we have moderation in almost all forums, and the ones that don't end up as cesspits of people shouting crazy ideas at each other and hence counterproductive places.
Giving everyone a voice doesn't necessarily mean we are bound to give everyone a equal voice in everything. Any weighing is in essence introducing a gatekeeper.