Whilst I agree that in the modern laws, it was a foul and a reckless tackle, it's more for the nature of the studs showing for me. I am thinking like a player here (not always a bad thing imo), but the whole issue with the 'trailing leg' in modern football is a bit of a falicy. Having slid to win the ball, at some point the trailing leg has to come through, it's just natural movement of your body to get both legs pointing the same way as your coming to a stop! You don't stop a slide tackle from front on with one leg pointing forward and one not, it just isn't how your body reacts.
To reiterate, it's a foul and a yellow card for the reckless tackle but I'm not having the trailing leg argument that seems to be popular these days. Scissor tackles from the side (deliberate bring through of the trailing leg) absolutely should be harshly treated but the danger in the tackle like Maguires is nothing to do with the trailing leg, it's the front leg that will cause injury. Having made Many of them tackles when you could do so,you don't even consider your back leg at all, it's all about the first impact on the ball/player