tldr: dying to bosses in non-endless runs is fun. dying to bosses in endless runs is not. ensuring you can reroll a boss in endless runs solves that nicely
so, here's how bosses are in pre-endless runs:
you have a build that's countered by a few bosses. you need a backup plan in case you encounter one of them. having a backup plan is part of the strategy and its almost always possible because pre-endless builds tend to be quite flexible. awesome.
in endless runs the situation is different. 99% of the time you are either going for baron+mime or idol+retriggers. both of them rely heavily on having a lot of the same card in your deck. getting that card debuffed means end of the run with absolutely nothing you can do. this is just not fun.
i think forcing ante 9 voucher to be director's cut (unless you already have it) is a great solution to this. it's a tiny change but removes a lot of unfun losses to getting unlucky