I think ladder parking is something that naturally happens due to busy life and other distractions but it's also a symptom other issues with the current system:
A- player's ladder position/achivement vanishes if they grind then stop playing for a month
B- (iirc, may be wrong) mm rating references shift over time, sometimes to lower values than the player had, so being very active fighting evenly skilled players will lower their rating over time, leading to avoidance (they only experience the drop IF they play)
to mitigate issue A, player's profile could show something about player's past achievements
- year, month and best rank color, top% the player achieved, ever
(reward intense participation at some point)
- year, month and best rank color, top% the player achieved and was able to consistently hold for 3 months, 6 months, a year?
(reward recurrence/consistency)
to mitigate issue B (if true), when ladder references shift at the end of each month or whatever, shift player's scores immediately, and/or do explicit MM rating rescalings or resets, that way people won't feel punished when they eventually get back to playing.