Why RCD is unallowed in TNC system? I think it is useful for preventing indirect electric shock

In a TNC system (unlike a TN-S or TNC-s) you cannot tell what is a legitimate neutral current, and what is an earth fault current using the PEN, as the earth and neutral path is common.

Therefore there will be multiple connections from the PEN to parallel paths (imagine TNC distribution and metal water pipes feeding a row of buildings - at each building the water pipe is earthed by a water heater.) so the neutral current is expected to follow these alternate paths   to some degree, so the live and PEN current are not in balance, even when all is working well.

If however you were very careful to only connected the PEN to earth at one point, then what you say would work, but the second connection to earth - the person, would unbalance the system.

In a TNC system (unlike a TN-S or TNC-s) you cannot tell what is a legitimate neutral current, and what is an earth fault current using the PEN, as the earth and neutral path is common.

Therefore there will be multiple connections from the PEN to parallel paths (imagine TNC distribution and metal water pipes feeding a row of buildings - at each building the water pipe is earthed by a water heater.) so the neutral current is expected to follow these alternate paths   to some degree, so the live and PEN current are not in balance, even when all is working well.

If however you were very careful to only connected the PEN to earth at one point, then what you say would work, but the second connection to earth - the person, would unbalance the system.