Half life is almost as confusing as statistics. A halflife of 5 years means 10% of the radioactivity is still there 16 years later. You'll still get 1% of the rems 33 years later, and that's if the materials don't migrate.

Alpha decays are still problematic for any material that can by aspirated or ingested. Demolition means flying dust. A leaky building means groundwater contamination. A lot of these substances are also heavy metals, so even without the alpha decay problem they are highly toxic.

Can you keep a building full of alpha sources water tight for five or six half lives?