There was no doubt he would be brought back in my mind:
a - Hasbro learnt the hard way 20 years ago about killing Optimus.
b - Optimus Prime is the biggest marketting element of Transformers, so once again, Hasbro wouldn't allow for the permanent loss of that 'asset'.
c - as BW Megatron once said 'you optimus' like sacrificing yourselves', so it is just something us old timers are used to now in Transformers. How many TFs series HASN'T had an Optimus die?
d - Megatron was only just resurrected a few scenes earlier, spoiling the fear that their deaths are final. THe moment he died, I immeadiately thought of the other Cube shard bringing him back.