Could preimplantation genetic diagnosis or genetic engineering get rid of diseases like marfan syndrome?

Probably yes: If embryos can be checked to see which ones have the Marfan syndrome mutation, then one can destroy the defective embryos and keep the other ones (although the other ones may have different disease-causing mutations not checked for). There are ethical questions regarding the destruction of human embryos.

Yes: An embryo could be tested for the Marfan mutation and only implanted if it did not contain that mutation. The same is true for a large number of other autosomal dominant or recessive diseases.