First, go upthread to see what I had to say about Guidance, and almost immediately, I got a response verifying the typical stance on it's use in social interaction.It's not even difficult though. Take a level 10 character as an example:
- +5 from attribute
- +4 from mere proficient or +8 from expertise.
- +1d4 from guidance (avg 2.5)
- that's already a good 11.5-15.5 towards that DC20 & we haven't even tossed in "and I help him" from the help action for advantage. Again the success chance is about double or better the batting average record brought up a few times earlier.
Second, +9 vs. a DC 20 is a fail case of 45%. So...better than a coin flip for 10th level character. Wow, amazing.
And finally, third, we shouldn't expect or require Expertise using characters to parlay with friggin' monsters. It's a time honored tradition and the foundation of the Interaction Tier....and a thing one does in a roleplaying game.