Why is it ok to only play once a month to avoid lp decay it's especially bad at d5

Everytime i get a d5 on my team i'm either playing normal or it's just the end of the month where they avoid lp decay so they can get their first season diamond border in FIVE FUCKING MONTHS i don't mind them playing only once a month only to avoid lp decay but it becomes a problem when it affects their games just by being rusty just remove fancy loading borders and ranks and implement an ACTUAL mmr system like World of Warcraft/Chess