I think DotA 2 MMR should have a seasonal MMR, and MMR reset every 3 month.
MMR must be start from zero.

for example :

MMR Season 1 : 1 January ~ 31 March
MMR Season 2 : 1 April ~ 30 June
MMR Season 3 : 1 July ~ 30 September
MMR Season 4 : 1 October ~ 31 December

remove callibrating MMR, so account buyer need to climbing their MMR again every 3 month.