Pryso, This has been beaten to the ground on multiple threads and of course viability depends on a record collection. If someone has very good stereo carts and a collection of mono reissues, they will get performance beyond that of a stereo cart and a mono switch, with a high quality mono cart. This is with vertical compliance to play a modern reissue in a reasonable manner.
Even if vertical rejection is 30dB - same as a mono switch, the 45° groove wall has two sides, just like a stereo record. With a stereo cart there is always a difference between channels - output, phase, etc. That difference gets combined with a mono switch.
I can't say it's worth it for any individual user, but companies like Ortofon and Lyra make high end mono carts.
Regards,