One of the core strengths of Bitcoin is in dynamic difficulty adjustment. If there are fewer people mining then the difficulty to mine new blocks will automatically decrease, which will in turn increase the value/revenue per block. And vice versa in the case where there are more people mining. So regardless of the state of the market, you will always reach an equilibrium point where it will be profitable.

The one asterisk that needs to be added here of course is that with sufficiently low difficulty, a double spend attack becomes more viable, but this is a somewhat overblown threat. It's low reward, extremely high cost, and 'easily' undone if the market so agrees. It shouldn't be ignored, because it is indeed a threat, but at the same it's also kind of a 'meh' threat.

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edit: And for those who are may not be aware. Even once the final Bitcoin is mined, miners will continue mining - something which may be counter-intuitive. Whoever mines the block will get no coins, but will get all revenue from the fees attached to whichever transactions are processed.