Just do your best plumber / car mechanic impression. "Gee, this codebase is really messed up, looks like I need to basically replace everything."
("That can't be true, that's what every other programmer I showed this to said. Are you all lying?" "No, we're not lying, this really is a screwed up codebase.")
One day I was thinking of Andy Warhol's film "Empire", which is basically one continuous 8 hour shot of the Empire State Building.
I thought it'd be cool to make a similar art film about your average programmer's work day. 8 hour shot of a programmer staring at the screen intensely, drinking coffee, scrolling through the code, and occasionally muttering "why the fuck doesn't this work?"