I might have as many as 10 messages left for a week or two because I, too, generally delete after reading. Only reason I asked is I tried to send a few PMs to members here and couldn't because their mail box was FULL.
processor time = 0 unless you are looking at your own mailbox, so you get what you wish for there. so let's do the math on storage and backups. every year, without fail (for several decades so far), storage only gets cheaper. go crazy and say we doubled every member's capacity - heck make it an entire meg per. and we have no idea what membership numbers are but suppose it is 100,000. i would guess this is a wildly optimistic number. last time i checked 100G was about $10 worth of storage unless you are using SSD...
I have a use for that $10! PM me for the address where to send it. :)
Storage is getting cheaper... but does it make sense to pay another $10/m/y (depends on hosting) or replace a perfectly good 500gb drive with a new 2tb when simple management allows use of that still in warranty 500gb for a few more years?
I keep butting heads with clients all the time over this issue of packrat everything. 12 to 15 year old files need to go archive. Sheer volume of info will eventually have an impact, though when one has to address it has been extended with the new capacities.
Folks will do what they will. Me? I'll just keep my PM space cleaned out in appreciation for the service, and acknowledging, regardless how little that account might be, I recognize there's a cost attached to it.
I delete any unimportant messages, but there are always some you want to keep. However, keeping old stickies on-site is pointless - the stickymail system is not searchable, so the archives aren't useful once you get past about a few dozen. Adding extra storage space does not solve that problem, irrespective of storage or CPU cost. Stickymail inboxes are simply not designed as a permanent store.
Using the "forward email to me" option is the best way of managing stickymail (forwarding as individual messages). I have currently one message in my inbox, but 2500+ messages stored in my email, all fully searchable.