There are OTB analogues to this
1. Suppose you both have two minutes left, and your opponent moves but forgets to press the clock. Do you tell them or pretend to concentrate hard on your move until their flag falls?
2. Or what if their mobile goes ding-a-ding-a-ding when they are soundly beating you and it is only a couple of moves to checkmate?
You can take different attitudes, but in neither case I would say it is unethical to take the win.
I have takebacks disallowed. I'd never ask for one and I don't see the point of them unless it's an unrated game, which I don't play. If I misclick I generally resign - it's not a big deal. If my opponent clearly misclicked (eg moves king instead of castling) and isn't obviously losing, and is decent about it I will offer them a draw.
Neither is unethical. On a clock, I usually wait a minute or so, then point to the clock to alert my opponent. Thirty years apart, I had two opponents let my clock run, and I think it is cheap and unsportsmanlike, but -- hey, it's part of the game. So if they want to play like Ty Cobb, okay, they will die without friends, -- but it's their right.
On the phone, I would let the TD handle it, but screw it, any guy who leaves his phone on deserves to lose. I hate those gadgets.
So, suppose in a tense endgame, I say, "Wait a minute. I see now I should have put my rook behind the passed pawn. So let's go back a few moves, change my move to 39. Rab2, and we'll play it from there." Would you say, sure, it's only fair? It would be unethical for me to take advantage of your lapse of memory on a central tenet of endgame practice. So, it's a better game if we reverse it all.
The existing rules are a good solution to the conflicts that can arise. If my opponent picks up his queen, then sees there is no good square to place it, so he puts it down and picks up his bishop, I'm not going to feel bad about calling him on it. Of course, if he lies and says it didn't happen, and I have no witness, c'est la vie. But we need to learn to play by the rules, and the way to learn is to follow them strictly.
I've simply disabled takebacks, solved!
"I thought I had something more to say." Pink Floyd. Or suppose, in a complex middlegame situation, your twelve year old oppoent leaves the tournament room, and following him into the hall, you see him in an intense discussion with his domineering father while looking at a laptop, and then he returns to the board, and promptly makes a move, which turns out to be losing. Is it unethical to accept the win knowing that he was unfairly influenced by his cheating father?
For Classical time controls, you really ought to be playing with move confirmation, especially on mobile. Then you don't have to worry about mouse slips or glitches or takebacks.
I am so happy to be in the "one percent" by some measure. Can you cite me to the verified statistics that 99% play by smartphone? Darn, left behind again. Granted, I still cut firewood for the stove, and wash dishes by hand.
I'm surprised to learn that there is even any wood left in Bolivia.
On the altiplano, none, but on the slopes we still have some, despite the burning.
@piscatorox : Where I play OTB chess, you loose immediately if your phone makes a sound - even if you don't take the call. So your second example is no example at all. I am not sure if I even could reject the win if somebody else in the room notices the phone.