I'm going to break one of my own rules and post here without reading every single post in the thread as I don't have the time for that tonight.
Just a few things to consider:
Dogs were domesticated approximately twice as long ago as cats were, and are naturally pack animals which most cats aren't, so comparisons on how they are treated can't really be made.
Studies have shown that the main reasons for the decline in song birds in the UK is nothing to do with cats, it's down to loss of habitat and the effects of intensive farming e.g. pesticides vastly reducing the supply of insects for them to feed on, modern large scale methods reducing nesting places in hedges.
There is a major difference in life expectancy of feral cats, stray cats and "owned" cats who are able to roam. The statistic quoted a few times above for a life expectancy of 2 years is that for feral cats, i.e. those born and living their lives in the wild with no human interaction. Stray cats (i.e. those who used to have a home) tend to live about the same amount of time after becoming stray, so if they become stray at 5 then their life expectancy is 7 as a very rough guide.
Average life expectancy of "owned" cats both indoor and outdoor is the same, around 11-12 years though many last until late teens or even 20..
Indoor only cats and "owned" cats who go out have approximately the same life expectancy, though tend to have different causes of death e.g. cats who go out have physical hazards like cars, dogs etc, whereas indoor only cats are more likely to get illnesses caused by being over weight or not having enough exercise, or ones exacerbated by stress.