SS in fresh water lasts as long as the boat in most instances. It should be inspected regularly for cracks in the swages or broken strands in the wire. Damage from corrosion is not an issue like in salt water
but stress hardening and cracking is. The head
stay should be carefully monitored for broken strands as it is subject to the most side loading.
Insurance companies will ask for a rigging survey and/or have a rigging exclusion if they are concerned about your rig.
You can re-rig the boat yourself cheaply with mechanical terminals like Norseman. Buy the wire and terminals and use the old wire as patterns. Use halyards as temporary stays when you bring down the wires to fabricate the new wires. After the first couple of terminals, you should be able to knock out fabrication in minutes per wire and re-rig the boat in less than a day. Use the money
saved to buy a Top Cimber or climbing harness and gear
to make going up the mast no problem.
Doubt that your boat was originally rigged with 316 wire. Production boat makersl used 304 for everything. 304 isn't bad, just less 'stainless' than 316. 304 is stronger than 316, btw.