Unless you leave it stored packed tight, you have decades of use out of a down bag.  Unless we're talking about only July/August nights, a synth bag has at most a 5-10 year life of it's stated temp rating.

Sleeping bags and anything that lofts for insulation should be stored fully lofted in the ubiquitous cool/dry place.

I pack my sleeping bag with a heavy duty garbage bag liner in the stuff sack.  On a long trip I might use two and stuff a spare or two.  
Once the bag is in the stuffsack I put all my weight on it and beat it about to get the air out.  Then I twist each liner bag and stuff the end down the side to secure.  No knots, knots in garbage bags become rips.
Then the compression straps go on and it's secure enough for me.

My preference for waterproofing everything that needs it is to deal with items individually or in small batches.  A single waterproof pack needs only one failure to get everything wet.  A bunch of waterproof sacks in a pack that's just abrasion resistant is better piece of mind for me.

My pack floats.  The only things in it that can get wet are things made to get wet, tarps, rope, stuff sacks.   One should be able to paddle one's pack home.   :D