What if everyone did that?
There's a common sense view on ethics, and a philosophical counterpart. The common sense view is that "What if everyone did that?" is a relevant comment/objection to someone's behaviour. The philosophical counterpart is some Kantian-esq view on universalisability.
Both are false. That one's behaviour would be bad if everyone did it, doesn't show that it is bad when everyone isn't doing it. Examples are easy to formulate: If everyone studied philosophy, we'd all starve and die. But studying philosophy may be valuable regardless, given that enough people are producing food. Morality doesn't require that we all do things that everyone could do.
One obvious real life example of this is peer to peer sharing on the internet. That the music/film/game/software industry would collapse if everyone pirated doesn't show that piracy is wrong. Not everyone does commit piracy, and so the industry won't collapse, or at least matters are not quite so straightforward. Piracy may be wrong, or right, for other reasons, but lack of universalisability isn't one.
Yet I feel that there must
Yet I feel that there must be a class of actions to which the 'What if everyone did that?' objection does apply (environmentalist considerations come to mind). Could we reach a definition of when universability matters and when it doesn't?
Thoughts on "if everyone did it"
Maybe part of the confusion here is that the things we do are not 24/7. In that sense, I would disagree about the philosophy example — we wouldn't starve if we were all philosophers, just as long as we (or nearly all of us) did physical work most of the time.
As for the peer-to-peer example, I think the "if everyone did that" does work as an argument, because one of the core assumptions of the argument is that fairness is extremely important, and that fairness can be defined as equality of opportunities. While low levels of p2p sharing might not cripple any economy, how do we decide what fraction of the population is allowed to do it? Once you've decided this fraction, you have perhaps changed the nature of what we're talking about, so that a lot more people would be okay with p2p. After all, if artists could predict exactly what percentage they are going to lose to non-paying file-sharers, they would be in safe territory, and this system might be considered fair if it were done by lottery. Still, the best approach is probably to limit all file-sharing to paying systems… but that's getting off topic.
So my feeling is that the "if everyone" argument is very effective and real — if it's coupled with a "because that would only be fair". It's a distinction between opportunity and action. Not everyone can be the president/prime minister/whatever, but in principle, everyone has the opportunity for it.