I notice high MMR Monkey Kings who are playing core don't really buy the aghanim's shard much, probably because it is 1400 gold and doesn't add any damage (and damage is really important for a position 1).
I hope they keep this shard concept instead of scrapping it just because it is underused though. All the shard needs are number tweaks/buffs rather than a redesign. Mischief is a really cool ability and it would be a shame if they gave up on it and redesigned the shard.
The buff could be: mischief invulnerability duration from 0.5 seconds to 0.75 seconds. That might make it viable.
Btw MK's invulnerability isn't like phase shift, it doesn't allow MK to avoid stuns or cc. It only blocks the damage of the stun or cc (e.g. you can still fissure to stun an MK who used mischief although it won't do damage). And I think 1400 gold for a poor man's version of phase shift for 0.75 seconds that doesn't block stuns or roots is pretty fair (although it can disjoint)
MK's winrate is also pretty dogshit right now (7th lowest on Dotabuff) so if they want to buff MK they could start from here.
Source: https://www.reddit.com/r/DotA2/comments/u8sjln/monkey_king_aghs_shard_concept_is_really_cool_but/