Good question. A point of diminishing returns is probably different for everyone because of unique goals.
The OP specified MC cartridges recs. I would suggest also looking at the associated phono stage. MC cartridges are very sensitive to loading and capacitance. It could be that a relatively modest change in a phono stage can yield a significant audible improvement. For example try an external stage to replace an internal stage. Try a SUT (if you haven't before). Even a different phono cable can have a significant impact.
As for cartridges, there is great value to be found in purchasing older/used cartridges then having them rebuilt. Remember that a cartridge is a mechanical device, and it does wear out over time. I chose a Denon 103D and then had it rebuilt to Ruby Level by SoundSmith. I could have spent more but the resulting sound is astonishingly good. This cartridge displaced a Audio Technica AT33ML-occ (2nd from TOTL at one time). Both were mounted into the same arm and table (Linn LP12 / ITTOK LVII) and same phono stages (ARC SP-10 MKII / AI M3a and M3b // Both with and without Cinemag 3440 SUT).
Several variables to consider in answering this question.
The OP specified MC cartridges recs. I would suggest also looking at the associated phono stage. MC cartridges are very sensitive to loading and capacitance. It could be that a relatively modest change in a phono stage can yield a significant audible improvement. For example try an external stage to replace an internal stage. Try a SUT (if you haven't before). Even a different phono cable can have a significant impact.
As for cartridges, there is great value to be found in purchasing older/used cartridges then having them rebuilt. Remember that a cartridge is a mechanical device, and it does wear out over time. I chose a Denon 103D and then had it rebuilt to Ruby Level by SoundSmith. I could have spent more but the resulting sound is astonishingly good. This cartridge displaced a Audio Technica AT33ML-occ (2nd from TOTL at one time). Both were mounted into the same arm and table (Linn LP12 / ITTOK LVII) and same phono stages (ARC SP-10 MKII / AI M3a and M3b // Both with and without Cinemag 3440 SUT).
Several variables to consider in answering this question.