Especially with woven dacron sails, most sailors simply dont know how to RAISE them and what tensions should be applied by the halyard (and/or cunningham); this leads to 'baggy' sails and a boat that aggressively heels over and is 'cranky'.
Another 'characteristic' of woven dacron sails is that the 3 strand 'boltrope' (inside a sleeve at the luff) typically shrinks/shortens over time and 'usage' .... and all thats needed to 'restore' a bagged-out sail is to simply 'readjust' the boltrope. Sailmakers usually dont like to readjust boltropes, as they would sell far fewer new sails.
Most times a boltrope readjustment and 'proper raising' by the sailor will enable a supposedly 'old' and 'baggy' woven dacron sail to be correctly shaped and longer-lived. Modern 'quality' woven dacron is quite 'robust' and dimensionally stable ... with periodic routine 'boltrope readjustment' such sails can be in service for a VERY long time ... and with good shape too.
Here's a posting from another forum on how to properly raise a boltroped cacron sail .... and how to assay and what to do about it if the boltrope has become 'shrunken'.
How to properly RAISE a woven dacron mainsail - SailboatOwners.com
.... see posting #1