Say I have three 1/1's. My opponent has
Slimefoot, the Stowaway and a Saproling. He's at one life. I attack with everything. Is opponent:
1) Dead because he can only block two attackers -> one goes unblocked and his life goes down to zero, or
2) Alive because his saproling dies, Slimefoot triggers, and he gains one life?
I strongly suspect the answer is #1 since Slimefoot triggers presumably never resolve if one is at 0 life (because of state-based actions), but I'm not completely certain. However if one of my attackers had first strike then my opponent can stay alive by blocking that first striker with the 1/1. I had a similar situation come up in arena, but didn't get to confirm how it works then.
It's 1 for the reasons you stated, unless option 3) he has 4 mana untapped and makes another saproling.