After reading this post a VEPP (Very Experienced Parks Person) emailed me:
"What a rookie mistake! There were quite a few options to deal with pedestrian traffic including permeable pavers. There's heaps of space for pedestrian access so even a chunky mulch wouldn't have been a problem. This looks like the typical "bells and whistles" mentality - ie we'll do a major revamp and forget about the existing natural assets."
Leaving the ground open around the tree is always the best option if you can achieve it. I think you could have done that here - even, if necessary, it meant people had to walk slightly around the tree. Would that be such a bad thing? It would send a psychological message that "trees are important and sometimes we have to go out of our way a bit for them."
It's always easy to find reasons why we can't do something. The real challenge is to find ways we can.