Asgard is the people not the place. Odin states this in the same film and I believe Thor reiterates it later on. As such as Asgard is still around then he wouldn’t lose his powers. And of course there is also New Asgard in Norway that we could argue is the (new) place as well.
Odin: Asgard is not a place. Never was. This could be Asgard. Asgard is where our people stand. Even now, right now, those people need your help.
In my opinion I don’t think Surtur destroying Asgard weakened Hela so he could destroy her. His sword actual goes straight through her, presumably killing her, before he plunges it into Asgard’s centre. And as we saw Hela was still incredibly powerful at that point. Surtur kills her before destroying Asgard, the place.