As has already been demonstrated by the permian extinction event, the biosphere can take a hell of a hit, and life will go on.
I think that you really have to understand timescales here. A 100 million years is a long time, just like space is big, really big. So that's a long damn time, and life will go on. intelligent life, maybe not so much.
as for why we haven't heard from anyone, why isn't the simple answer not the best ?
Remember how space is really big ?
if there's no FTL travel, and it's likely there is not, then HOW would we hear from someone ?
It would be an exceedingly difficult thing for the intelligent civilization in the Andromeda galaxy to talk us, and us to them.
First of all, there's the 2,000,000 year latency, and then the amount of power you would need to transmit that signal, etc...
I'm not worried. There's intelligent life elsewhere in the verse. I'm pretty sure we're not going to hear from them any time soon, if ever.