Replacing Relay with Triac
I have been using relays controlled by microcontrollers in my other designs on daily basis, but due to space constraint, I am thinking of using Triac in this design.

If you are space constrained I will advice you to use normal relay(Electromagnetic) rather then one with triac based because in later case you have to use opto-coupler for galavanic isolation + snubber circuit for triac + snubber for opto-coupler as recommended in datasheet on page 4.

My expectation is 5 years mean life expectancy with 8-10 Hrs of operation/day

I can't tell the exact Life time of solid state relay(one with triac based)but surely it is more then the electromagnetic relay.So may be you don't need to take concern about it.