The sun won't stay the same size forever, and neither will our galaxy. In fact, there is a small elliptical galaxy in the process of colliding with the other side of our galaxy. Our galaxy could potentially absorb this galaxy. But not all galaxies have a supermassive blackhole at the center, only spiral galaxies and some irregular galaxies that are the product of galactic collisions. Also supermassive black holes are currently only a theory for what is at the center of our galaxy and those like it. But if there were a supermassive blackhole at the center of our galaxy, eventually if time permitted, it could pull everything in our galaxy into it's singularity. We're in the schwarzchild radius of that supermassive blackhole.