To an extent, they have already relaxed the engagement clause, as the last eratta to divine challenge made it so that failing to engage didn't prevent you from challenging next round.
Personally, I think the engagment clause is pretty much perfect - it reinforces the theme by discouraging the "mark and run" mechanic, but is relatively simple. Now, if only sanction worked the same way, rather than having 2 seperate end events for the two otherwise (nearly) identical features... having one that lasts until "do not engage" and one until "end of next turn" is a real pain, especially since it requires seperate tokens to give monsters.
Though, I suppose one could argue that the powers that sanciton all within "close burst x" might be too powerful is they challenged instead... as concievably you could have them all challenged for an entire encounter. On the other hand, given that the use of the challenge is an immediate action, if one monster eats the challenge, the others can just shift away means that it would be challenging to keep them all engaged... still, by having the paladin shift/move after them, you could potentially stretch the length of the divine mark.