I see many comments like this. The problem is not understanding that Number of K/D/As or winning percentage is not the measure of your skill. It's MMR/ELO.
ELO, the basis for MMR is designed to match you with similarly skilled players (MMR) and it is expected that your win rate is near to 50%, because you're playing people of your same skill. As you get better, your win rate will improve to 52, 54, 55% but then your MMR will improve and you'll get matched with better players according to your new MMR. Thus, your winning percentage will come down to 50% +/-, while still at your new ELO .
Number of K/D/As or winning percentage is not the measure of your skill, it's ELO. One could have a 50% win percentage playing against beginners or against Challenger payers, it's your MMR and competition you face which reflect your skill.