A: If John comes to the party, Mary will be happy. (So there is a chance that Mary will be happy.)
B: But John isn't going to the party. (So your argument is invalid.)
That's what the subjunctive is for. If A had said “If Jon came to the party, Mary would be happy”, ...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one.
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.