How long is the life cycle of a bicycle? This may not be a particularly practical question, but I wonder how often do you find yourself looking for a new (or another) bike? There are, of course, two main aspects of it:


Mechanical decay. If you drive a car, you know to lube it every 3000 miles, rotate tires every 7500 miles, flush the fluids every 30000 miles. If you are reasonably serious about riding a bike, you'd place yourself at wherever you are comfortable in terms of maintenance from being unable to adjust your saddle height to re-assembling your own spokes and hubs, but you'd do some maintenance, anyway. Most components would have reasonably well-defined span they will serve, and would probably start falling apart after this many miles, even with reasonable maintenance and repair. What would the range in time and/or miles be for commuter bikes? road bikes? mountain bikes? Or are the modern bikes of say $1000 and upwards basically perpetual, unless you ride one heads-on into a concrete wall?
Needs change: you outgrow your bike, your riding pattern changes, you switch from group rides to triathlon, you move to a new area, you hand down your bike to your younger sibling, etc.


I have a gut feeling that the latter is far more likely to happen than having the bike used up to its full mechanical resource. Any opinions?
Let's try to steer this into some kind of sensible question with the perspective as seen from a typical bicycle shop workshop and what is on the road. 

In a bike workshop you can find yourself working on bikes that can be up to fifty years old with a large quantity of them being more than ten years old. Some of these bikes have not been out of the garden shed for the last five years and have suddenly came into use because the owner's son or daughter has started riding.  Therefore this question very much depends on how often the bike gets an airing, how it is stored and how much it gets maintained.

For the person using their bike every day for commuting, the lifetime of the bike is an economic question. After five years or so, even with brake block changes, new cables, maybe the odd new set of sprockets, new tyres, replacement chains and plenty of t-l-c, the bike will suffer all-over wear that costs almost as much as a new bike to put right. Wheel rims will wear through, saddles will get torn,  bits of 'Shimano plastic' will go missing and the whole drive-train will get worn.

Fixing these problems will typically necessitate another round of consumables, e.g. chain, cables, brake blocks and tyres. All of these are more expensive as spares that you have to put time into fixing or pay a shop to fix. Meanwhile, at OEM prices, pre-fitted to a new bike, a better deal can be found. The existing bike has 'depreciated' to become a liability much like how an old car does. 

If you look at the cars people drive you will notice that there are not too many 'Ford Cortina' models on the road. (UK) There are not too many 'Ford Sierra' models on the road either. Yet, there was a time when every other car was one of these models. There are one or two of these left and you do see them once in a blue moon, so clearly it was possible to service them and keep them on the roads forever. However, that did not happen for 99.9% of these once popular Ford cars.

Now have a look for what people actually ride. There is a reasonable selection of twenty year old models on the roads used by people on the commute or taking the dog for a walk. However, depending on where you live, the majority of bikes are recent, i.e. less than ten years old. Half of those are very new, less than three years old. Most of these 'daily riders' are well on their way to being used up as a 'mechanical resource'.

As for your point 2), in the UK many bikes get abandoned by their riders when they take up driving. Before people get to that age they do go through a series of bikes that they grow out of. Competitive riders are different again, typically they have a fleet of bicycles and a ludicrous amount of spare parts left over from upgrades.

To summarise, a bike will have a lifetime of approximately five everyday-riding years before it gets shot to pieces. This lifetime can be extended indefinitely through new components and diligent maintenance (or instantly shortened in the case of a crash). In reality people can ride bikes long after the bike is past its best, they can also put the bike into storage for long periods of time and they can also get into the sport (to purchase many bikes that don't necessarily get worn out).