Plus, most large companies care about profits. Not feelings or the death of a lowly worker. If you boycott the source you'll have to boycott the whole chain of things, and it'll have to be widespread and immediate for them to care.
I just don't see it happening. We're stuck with this timeline it seems :/
Still it is not the way to change things:
"All in all, I think it is a mistake to defend people’s rights with one hand tied behind our backs, using nothing except the individual option to say no to a deal. We should use democracy to organize and together impose limits on what the rich can do to the rest of us. That’s what democracy was invented for!" -- Richard Stallman
Can you give some examples? Google don't run an online marketplace and they don't have huge warehouses or a a distribution network so I can't see how their business practices can be "similarly bad". I'm sure they do have bad business practices but I doubt they're in the realm of the inhumane conditions Amazon workers have to endure.
In Poland however, unions are often frowned upon because of Poland's past, although there are one or two small workers union that have been helping the warehouse employees.