I can see myself playing in the 40k setting without using the GW rules.
I don’t think I could see myself not using SM models though.
They are so entrenched in the whole setting that nothing else would seem right to me.
If the aesthetic was right I would happily use other models as opponents, but I don’t think I would want to play in the Grimdark without GW SMs.
Same with my fantasy games, the setting has a certain feel and some fantasy miniatures would not seem right.
Though I would and indeed do use many different models from many companies as long as they seem to fit the feel of the setting.
When I did my 6mm sci-fi the look was less important as you could have things from other worlds, for me the most important thing was the size.
Did the various models look right in terms of size next to each other.