But then 10 years down the line you'll be scrambling to implement Master Data Management, where you'll try (unsuccessfully) to merge user profiles and data.

I'm very anti-microservices, and rather than implement them for organizational reasons (read: teams unable to work together), I think that the most important business objects need to be maintained at a single point. Managing user identity at a single point is a valid reasons for microservices; like Google does with gmail/search/drive/storage/cloud services/youtube/android/etc.