Actually, there are examples of transitions from non-metal to metal and vice versa; In Extremo immediately comes to mind as an example of the former, and Kiske from Helloween as an example of the latter. Clapton, for example, played music ranging from hard rock (perhaps not metal, but metal didn't really exist back then) to blues and even almost pop pop music :)
And death and symphonic are not the biggest difference between subgenres; take, for example, gothic doom (like My Dying Bride) and some kind of folk like Korpiklaani...
Cradle Of Filth - Nymphetamine
Added 7 minutes later

A chart on the topic, it made me smile. Although I don't agree with everything. And the alcohol chart is still cooler)
To sell my soul