> with a track record of basic competence.
You need to ask yourself how that track record is constituted ie what metrics decide basic competence. Hint: it's not anything that resembles an objective measure of ability or diligence or literally anything. Another hint: it's *extremely* common for PIs to pawn review duties off on grad students - I review papers for my advisor all the time and I have been since my first year.
> To reduce the incentives for nepotistic anti-scientific behaviour.
This is the thin veil that everyone hides behind to cover up the greater harm of inscrutability^1; everyone knows what everyone else is working on in any field. At this point in my career (I'm towards the end of my PhD) I could tell you which group wrote the paper (if it's in my area) just by reading the abstract.
And if you're thinking to yourself well what about new entrants into the field? There's no such thing - you do not learn XYZ hyper niche thing without being part of a long lineage of forebears doing XYZ thing. And if it does so somehow happen, like a student is doing something completely different from their advisor and so they're not trying to publish in familiar venues, well guess what they get face looooong odds getting their work accepted (ask me how I know :)
^1 Like come to think of it, recently I've been reading that the conservative judges on SCOTUS think proceedings should be closed. And you could make some sort of similar nepotism case about that, or some other conflict of interest type thing (in favor of closing the proceedings) but we all recognize that fundamental to a free and fair process is that it's open and auditable.