Sounds like someone needs to read Manifold: Space*, but seriously:
1. Silicon instead of say, a lattice woven from near Planck-scale wormholes or some other loosely plausible but way out in distant hard sci-fi territory.
2. Children in 50,000 years implies people still die enough to need enough replacement to use the plural, which is a weird niche of "our tech got us this far, but no further, yet we didn't get wiped out for some reason" type doomsday scenarios.
3. Not only are they living mayfly lives at this point, but they've got an AI god (obviously not a Strong AI God) which can't (or won't) try to even teach them how to make things to fix anything which breaks.
4. Tech tends to break as we are developing it, and then if you don't maintain a built-in obsolescence there is no reason to assume we'll be hard up for self-repairing stuff that just doesn't break at all within a few centuries, much less a few kiloyears in the future.
5. Cybernetics kinda implies obvious implants, we're hitting that stage now, dear god what is with the apocalyptic mindset man, how are these people smart enough to reach that point in the future while being so stupid?
6. If humanity is done in by entropy it will be a long time from now, we've got great big gaudy stars sitting around still, we're smart enough to get by on the trickles of energy from red dwarf stars for trillions of years, and then we'll have some time before we reach the "how can we locally reduce entropy again" type of problems. Heat death isn't a billion year problem, it's a trillion year problem at the low end.
*Stephen Baxter, part of the Manifold series, Space is goddamn depressing, Time is confusing as shit but magnificent in scope, Origin is weirdly touching with some super bummer moments but it isn't as bad as Space--though what is, I'm not even sure watching a baby elephant trying to wake up it's dead mother is as soul-crushing as the ending of Space--and they're a huge mood switch from the Xeelee Sequence.