So long as your temps are decent and voltage isn't crazy it shouldn't decrease the lifespan of the cpu by any meaningful amount. You'd be due for an upgrade long before. 75c is 75c whether on stock cooling and lower clock speeds or overclocked with a better cooler. A water cooler isn't necessary at all. My 4690k is air cooled and it hits vcore voltage limits (for what I'm comfortable with) before temp limits. Some don't like larger air coolers, I don't mind the 'look' and as a plus it's quieter and nothing to potentially leak, less parts to fail than an aio cooler.
Seems like for just as many people as I see who love their water coolers, I see just as many with failing pumps, higher than expected temps, mounting problems, issues setting it up properly to make sure pump speeds aren't being reduced etc. Just not worth the headaches but that's my opinion. A custom cooling loop is a different story and can be much more effective, aio's (even dual rad models) seem a bit crippled in performance by comparison but then they're a lot cheaper than custom loops too.