I would think newer sats will have collision avoidance capabilities and older ones will just crash. Maybe even clean up sats will be developed to collection them
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At orbital velocities, you gotta know it's coming to be able to avoid it.
And a orbital velocities, the untrack-able stuff can still kill a satellite.
If your non refuelable sat is good for 6 months it probably no longer makes sense to launch it.
There is no incentive large enough for cleanup (it's expensive, nobody can/wants to pay, and there are a lot of objects)