this isnt true. i was 1500 at the start of last season, but after taking time to actually get better at the game i made it to 2500 in less than 2 weeks. and placement matches do not determine what rank you will be placed in ironically that is why i think they should get rid of every season placement matches. what determines if you have gotten better is if you are starting to get huge winstreaks during the season.
if your gameplay is getting better then you should have no problem climbing out of bronze. it is that simple. if you are having a lot of trouble climbing out of bronze then maybe you are still a bronze player. and that is ok.