Culturally, the engineers and management who created the mess should not be demonized. The situation is the enemy, not co-workers current and past. (Your private opinions can be unvarnished.)

Always steer towards how things should be looked at going forward: “we’re making this better for everyone as we add features/remove bugs”. “It probably looked like a good idea at the time” is a phrase I use a lot.

Newbies will see all the crap sooner or later. Knowing that they have more-senior allies in a shared battle, and having some ability to do stuff beyond fighting the crap, will help keep them on-board and engaged.