I've been part of three teams who attempted monolith->microsystem transitions. Two succeeded (though at wildly different costs in terms of engineering time and delay) and the other was abandoned after person-years of effort.

The common aspect of the successful migrations was their incremental nature: rather than "killing" the monolith all at once, there was a careful and gradual migration of performance-critical sections into services running atop dedicated machines/storage/etc.

Neither of the successful moves happened all at once, or indeed ever 100% replaced the monolith.

It wasn't just a question of planning, either. The failed migration had a team of three engineers spend ~six months writing detailed component specs, migration plans, etc. The business simply couldn't stop (or even maintain the status quo) to let them build the shiny new V2, so it kept getting pushed out and restarted long enough that the plans and specs bit-rotted and the whole thing got scrapped.