What do you mean, don't delete them? If you want to fill a map with.  purely historical data, add it to OpenHistoricalMap. Following OSM guidelines, removing objects that no longer exist is in fact the right thing to do. 

In cases where there was relevant data left over (address tags etc.) I retagged the businesses with the "disused" lifecycle prefix, so those aren't gone, but empty objects with only lifecycle prefixes on them would only clutter the database without contributing anything useful, so I chose to remove them. 