If covid vaccine effectiveness decreases after 6 months. does that mean we will be unprotected like we were never vaccinated? will chances of severe illness and death in 6 mths be just as high as if we never were vaccinated?

Normal course of virus evolution is that it becomes less virulent but more infective over time. That means higher risk of contracting a much less dangerous form of the disease. Couple this with the vaccines you have taken, or natural immunity from previous infection, and you get what we are already seeing, slightly increased reported cases with dramatically lower hospitalizations.

Vaccination provides effective protection against serious illness and death, even after 6 months. It would be prudent to get booster doses as advised by your doctor. For information: https://www.cdc.gov/media/releases/2022/s0715-COVID-VE.html