Sounds like Thanos is working well for you, so in your position I wouldn't change anything.

There are a bunch of other reasons why people might choose Mimir; perhaps they have out grown some of the scalability limits, or perhaps they want faster high cardinality queries, or a different take on multi-tenancy.

Do remember Cortex (on which Mimir is based) predates Thanos as a project; Thanos was started to pursue a different architecture and storage concept. Thanos storage was clearly the way forward, so we adopted it. The architectures are still different: Thanos is "edge"-style IMO, Mimir is more centralised. Some people have a preference for one over the other.