Is declining to win to burn time on the clock punishable?
I would say that this is not in violation of any rules. By taking more turns than necessary, Alice is giving Bob additional chances to draw outs to this situation, so it is not unambiguously to Alice's advantage to draw this out. And Alice is drawing two cards per turn, so Alice only has 15 to 20 turns before she either decks out or plays Entreat and wins. If each player plays at a reasonable pace and takes few actions each turn, they could probably finish with more than that much time on the clock. If Alice slows down her pace of play to make the game last longer, then it becomes a clear case of slow play, and probably stalling.

In addition, each player has a fast way to resolve this situation if they believe it is not to their advantage. If Bob believes he would be better off if Alice won quickly, he can always concede. And if Alice believes she would be better off winning quickly, she can do so as described in the question.

Furthermore, there are perfectly valid strategic reasons to play slowly like Alice is doing here. For example, she may suspect that Bob has a Dovin&#39;s Veto in hand, and be trying to find a different spell to draw it out without risking the Entreat.