I think you're confusing forever with a really long time. (1) Asteroid's velocity is eventually overcome by external forces. (2, 3) Structure decays. (4) Bodies converge or diverge. (5) Radioactive decay. (6) You are observing a high energy system. (7) Etc.

I don't think I'm confusing forever with a really long time; I'm just assuming they're effectively the same.

(Aside: nothing exists forever - even time crystals - because eventually the universe will be a bunch of infinitely far apart black holes or something. So I'm not sure how that assumption is relevant.)

The distinction still makes sense, I think. There are particles with a decay time so long they're practically stable (e.g. bound neutrons) and then there are stable particles.

The examples you gave above are forever; I just started assuming things like eternity and I can go on to assume no external forces/stuff in the way. (So not disagreeing with your initial statement, just adding.)

For me, the difference is whether these things a) have an external force being applied to them, or b) are slowing down and are only going on "forever" because the rest of the universe will collapse before they come to a standstill.

For example, even ignoring the fact that the sun is going to explode before the universe does, the earth's rotation around the sun is gradually getting slower and slower. If the sun, and the universe, did last forever then the earth would eventually come to a halt.

In the case of these crystals, my impression is that even in that hypothetical never-ending universe, the movement in these crystals would never slow down.

Actually there is no such thing as a stable orbit in Eindsteinian mechanics (except for 2 point masses orbiting eachother. There is no such thing as a point mass in reality). So the energy conversion from speed to distance and back is not 100% efficient and an asteroid (or the Earth, for that matter) is doomed to eventually fall into the sun or fly away from it forever. It cannot keep circling forever, even in a theoretically perfect situation.

In addition to that, the tidal forces will compress and decompress the mass of both objects, which effectively converts potential energy (the distance between the two objectds) into heat. Even though the moon is on a trajectory to eventually leave earth orbit, every tide that comes in and leaves lowers the rotation speed of the moon. While the moon has no liquids on it's surface, which makes the effect nearly invisible, it does have tides (slight variations in the angle and magnitude of gravity at it's surface) which move objects around and the energy for that does indeed come from orbital decay. Similarly, the earth loses orbital energy to the sun.

Yes. F=ma. There's no v in there.

yes there is, it's right in the v'(t)

My point is that acceleration requires force, but stable velocity requires no acceleration, hence no force.