A relay should last a LONG time if it's used within it's specs (and if it's from a reliable manufacturer). A rating of 100,000 cycles seems common.
I'm sure redundancy is possible, but as has been said it's not "easy". If you put two relays in parallel it will still work with one relay is stuck open, but not if one is shorted. Two in series are the opposite where it won't work with one stuck open. And with the relay-count doubled you've doubled the probability of a failure!
I only remember seeing one outright relay failure in my life and that was a horn relay in an old car. (A rather harsh environment under the hood.)
Where I work we have a device with 16 relays. Once in awhile one is returned with a failure, but it's never returned because of a relay failure. Sometimes the relay is out-of-spec (the on-resistance is too high and our automated test checks for that) but the relay was still working and it was returned for a different reason.
A solid state relay can "last forever" but they tend to be less "electrically rugged" and can burn-out instantly with excess current or voltage. Also, AC & DC solid state relays are not interchangeable and sometimes there is a minimum voltage & current.
You can also find SSRs that operate from 5V at low current so they can be directly driven from the Arduino, whereas regular electro-mechanical relays require a driver circuit for the coil.
There was a recent post where someone was using an SSR for LED Christmas lights and there was enough leakage with this low-current load that the LEDs would dim instead of turning off.