The plus point is that you have a captive audience and a lot more punters than would normally be in the venue.
It happened to us a couple of gigs ago, it was a couples wedding anniversary in the pub/restaurant and we had a great night.
For the sake of keeping a happy band and the landlady not willing to budge on only wanting a three piece, I personally wouldn’t bother with the pub, it’s only a gig and not worth the hassle.
The other side of the coin regarding your last sentence is a lot of the new stuff has no relevance to folks who were really into the music produced thirty years ago.