In an interview in the recent article Mars Has So Much Radiation, Any Signs of Life Would Be Buried Six Feet Under physicist Alexander Pavlov of NASA's Goddard Space Flight Center tells that he and his research team found out that amino acids are destroyed by cosmic rays at much faster rates than previously thought.
At the max. 6 cm depth that Perseverance drills down it would take only 20 million years to destroy amino acids completely.
From the abstract of the research article Rapid Radiolytic Degradation of Amino Acids in the Martian Shallow Subsurface: Implications for the Search for Extinct Life:
Our experimental results suggest serious challenges for the search of ancient amino acids and other potential organic biosignatures in the top 2 m of the martian surface.
Emphasis by me.
So now it appears that the samples taken by Perseverance will have only a very small chance to contain biosignatures, isn't it justified, regarding future and already spend costs and time, to send an extra rover equipped with the ExoMars drill unit that has a far better chance in getting samples containing biosignatures, prior to the Mars sample return mission ?
This rover wouldn't need all the instruments that Perseverance has since the ExoMars drill unit embeds a miniaturised IR spectrometer for exploration of the borehole, and it could travel right to the places already determined by Perseverance to have the highest probability to get biosignatures.
Or do we have to wait with great expectations until at least 2033, to find out that indeed the returned samples don't contain any biosignatures and then wait again for many years for the results of a next mission with a rover that finally has the capacity to drill down to at least 2 m. ?