Heel strike running is bad because multiple lines of research over the last few decades have proved the many verifiable aspects of heel strike running that are primarily responsible for nearly all running injuries vs forefoot running.

How Heel Strike Running Causes Plantar Fasciitis
Plantar fasciitis can be added to the list of injuries caused by heel strike running because the higher-than-normal amounts of impact on the heel at touchdown was found to cause plantar fascia degeneration and fragmentation as well as reduced thickness and atrophy of the fascia. This is why heel strike running causes plantar fasciitis to turn into a chronic condition as compared with forefoot running.