Farrockgrandma:
Regarding your first point, researchers from a major Israeli university developed a mathematical model to predict the future carrier rate of the Tay Sachs’ gene in the case that no two carriers ever marry. The result was that yes, the carrier rate will increase , but will take at least 500 years for it to be worth any consideration, and even then the effect will be miniscule.
Regarding your second point, you are absolutely correct, diversifying the gene pool is extremely helpful in reducing the incidence of genetic diseases.