The reason to soft delete is to preserve the deleted data for later use. If you need to not query that data for a significant amount of the system use that 75% soft deletes is a performance problem, then you either need to move the soft deleted data out of the way inside the table (partition) or to another table entirely.
The correct thing to do if your retention policy is causing a performance problem is to sit down and actually decide what the data is truly needed for, and if you can make some transformations/projections to combine only the actual data you really use to a different location so you can discard the rest. That's just data warehousing.
Data warehouse doesn't only mean "cube tables". It also just means "a different location for data we rarely need, stored in a way that is only convenient for the old data needs". It doesn't need to be a different RDBMS or even a different database.
The reason to soft delete is to preserve the deleted data for later use. If you need to not query that data for a significant amount of the system use that 75% soft deletes is a performance problem, then you either need to move the soft deleted data out of the way inside the table (partition) or to another table entirely.
The correct thing to do if your retention policy is causing a performance problem is to sit down and actually decide what the data is truly needed for, and if you can make some transformations/projections to combine only the actual data you really use to a different location so you can discard the rest. That's just data warehousing.
Data warehouse doesn't only mean "cube tables". It also just means "a different location for data we rarely need, stored in a way that is only convenient for the old data needs". It doesn't need to be a different RDBMS or even a different database.