If implemented, a rank decay is welcome, it will ask you to keep somewhat active to keep your rank.
Without decay, whoever gets the aimed rank will stop playing to avoid any risk of losing it.
Then, there is the problem of throw away a match with alt or friends accounts. It can be mitigated with a rule that forbid the players of the two teams to meet again together for a given amount of time, enough time to discourage or make not productive this kind of cheating.