If the Amiga didn't EVER exist, I would have continued with my Atari ST, possibly going TT and then moving as I did last time onto PC. (I went PC in '96, because Amiga was dying out, and the price of a top flite Amiga was more than a superior specified PC).
I did also have an SGI Octane for a while, but once I had voodoo 2's etc in my PC, the PC kicked the Octane's ass at 3D performance, so I got rid of the unit (but kept the tasty 20" Granite finish Monitor and used it on my PC)
If I didn't have an Amiga NOW, it'd make no difference. I don't use mine anyway, I use WinUAE, so I suspect that if Amiga's had existed, but I just didn't own one anymore, I'd still be tinkering around with WinUAE. Apart from the fact my Amiga is back in my country of origin, it was a pain in the ass to connect it all up and make space for it on my computer desk (meant tidying up and nasty stuff like that) and also meant I had to dig through spiders webs in order to disconnect the Monitor from the PC to connect to the Amiga, or hauling another monitor out of the spare room.