> And if you ever need to read anything even once database without indexes is slow

This may be true in most cases, but not all. It just depends on your access pattern. If all you do are full table scans (possibly feeding to hash-joins), you won't benefit from indexes at all!

The whole point is that indexes do not auto-magically improve performance with no downsides. If they did, we could just index all columns and call it a day!