Ultimately, what makes the storytelling decision a good one in Johnson’s eyes comes down to Helen’s motivations for doing it, and the way it reflects Bron’s own speech about disruption earlier in the film. As he put it:
“Miles himself says it with his disruptor speech. It’s easy, to start breaking these little glass things … you start breaking bigger stuff, people get a little apprehensive. The question is ‘are you willing to break the thing nobody wants you to break?’ … You can’t just break a more expensive chandelier; it needs to be something that creates exactly that effect in you, of ‘I don’t know how I feel about this.'”
For all Bron talks about wanting to disrupt the system, he and his friends are people who clearly benefit from that system and don’t have any wish to see it destroyed. Their acts of supposed rebellion against the status quo are nothing more than them trying to grab more power and make more money. Burning up the Mona Lisa, meanwhile, is an act that actually is bold and impactful, which takes genuine audacity to go through with. (It’s also a bit of a tragedy, but that’s neither here nor there.)
As Johnson explained:
“The whole thing at the end was putting his words into action and using them against him, and him being actually horrified because he’s full of s***; he doesn’t want actual disruption, he doesn’t want the system to be broken. That meant we actually had to go there.”