In the daily retention you can set how long to keep the logs and indexes but not re-index them.
We have the logs and we have the indexes. Normal search in the logs view will work only if you have indexes but if the log files still exist you can open them (but one at a time though) and they could still be re-indexed any time.
So you can keep the indexes for X days and that is the amount of day you can search in the logs view without opening the log files one at a time. The logs are stored for extra Y days, which mean you will have long retention of X+Y.
The amount of days that I can recommend could vary over some factors. I think that using the retention by disk space is fine, just make sure that 10% is above 10 GB or something else you're comfortable with.
Kind regards, Amir Senn