Refurbishing everyday bikes I'm looking to get 3 family bikes functioning as well as possible. I've replaced all the cables, but now want to get the gears as close to new as is sensible.
All bikes old >10 years I would guess:Trek 4300, Kona Lana'i &amp; Ridgeback mx24. 
Initial Q is, apart from the chain &amp; cassette, which gear  components degrade the most? Rear mech, front mech, shifters &amp; chainring.
Trying to do a cost-benefit analysis.
Ta
Depends how they've been stored.  If they were inside a garage or otherwise dry, then that's a lot less work.  If they were outside or damp, everything will be that much closer to EOL.

Gear changes and brakes: if there's been moisture in the cables then they can suffer but housings and inner wires can be replaced.

Tyres and tubes can degrade in heat.  Oils and greases will thicken and congeal so an all-round service of bearings can be in order.

Finally, why were they unused for a long time?  Was there a fault that was too hard to fix at the time?   Look for function. 

On the other side, sometimes a good wash, and a lick of paint can make an old bike appear very much nicer too.