Absent stainless steel rigging could you rig the boat with alternate types of rigging? Are there any manufacturers of actual rope rigging still in existence?
I'm using galvanised steel wire - a little unusual these days - but stainless is beyond my budget (5x the price) and galvanised can be made to work a long time (many decades) if looked after well enough (longer than the much more popular stainless actually as stainless is limited by fatigue and can only be relied upon for maybe 10-15 years depending).
My sails are fairly small by modern standards and it might be possible in theory to use high enough quality natural rope (but rope has become really quite high tech these days... so when you say manufacturers of rope I assume you mean traditional stuff, not the new high tech stuff) - if memory serves I calculated my sail loads might approach up to several tons per sail at the upper (normal) range of operation. Breaking strain on the wires is around 7 tons, working load ~1.5 tons - 8 shrouds and 4 stays per mast (for around expected 750 ft of rigging, give or take - I don't seem to have consumed quite that much wire, perhaps as other components take up some of the length - turnbuckles, shackles, chain etc).
Could I make up my own ropes from natural fibers and rig it? Perhaps - but the extra weight aloft might negatively affect stability, and I can't even imagine how long it would take to make all the rope up to a standard good enough (considering I can't make rope myself right now and would have to work it out from a book and trial and error...)
Could I do it in a timely fashion in a survival situation without outside inputs? No, realistically not. I carry some spare wire - and could salvage other materials in theory - but very much doubt I could re-rig this vessel from first principles in such a difficult situation.
What about a replacement for all the fancy modern sail materials. Do they still exist?
I assume that the very most traditional of sailing vessels might still use old materials - but for both rigging and sails I think it's a pretty safe assumption almost nothing out there is still doing so. The maintenance burden is much too high and the skillsets too obscure to do it for any reasons except historically correct tall ships. Even then - wire rope has been around for quite a while now.
I do not currently have the materials, tools or expertise to fabricate replacement sails (or even to substantially repair my existing sails) although I do have a book about making sails. Modern sails - for cruising use - where you run them down and don't start with super costly high tech ones as for racing - I guess can last 10-15 years if looked after? (I have a basic sail set in reasonable condition and a couple spares that don't fit properly as they were grabbed cheaply from a boat that was broken up but could be adapted or used to patch the rest). New sails are too expensive - if I need more - I need to make them (or find good used ones going dirt cheap).
You're talking about 1500 sq feet of material for a set of sails in my case, if memory serves (though 1200-1300 would probably do, the fifth sail is optional and currently missing anyway).
Even if this vessel could be rigged with traditional historic materials I'm pretty sure you'd have to be very careful trying to sail it that way (and most larger boats I think you could forget it due to the much larger sails and hence loads used these days - I have numerous and small sails compared to many modern vessels)? Or you'd need an awful lot of faith in the capabilities of those materials in the context of the conditions you were using them? Boats and sailing have changed an awful lot over the centuries...
I think on the whole I don't see most of the existing boats being operable a couple of decades after collapse (except by the most dedicated and creative of scavengers - and note many people running sailing boats solve problems with their wallets first and foremost, and expect the world to work that way).
I also think there is only a handful of people alive in the world even today - before collapse - who could build sailing ships using historic methods and more primitive materials? Mostly, they're going to be older people...
If collapse occurs to the floor I expect, I envisage people will need to largely rediscover boat building and sailing.
And if my vessel is ever used as hypothetically intended - I expect one day to run it up onto a beach for the last time and to leave it there to slowly rust away - or some similar ultimate fate.
Fibreglass hulls will last a long time, but steel tends to rust. I have tools, expertise and materials to carry out a certain amount of even structural repairs on the move without external assistance - but without fresh supplies of modern coatings and chemicals - that's a losing proposition longer term - and entirely depends upon the continued operation of multiple modern pieces of equipment (mostly Chinese imports...) and fossil fuel to generate the electricity needed (my renewables once finished will still not produce enough power to cut and weld steel).
Given systematic and widespread collapse, I do not think anywhere near as many technologies will be capable of being kept in operation as people seem to think?
Again - it's easy for someone to sit in an armchair and say "of course that won't be lost, of course we'll have X or Y" - but of actually be capable of ensuring that - or even knowing anyone who can ensure it? Good luck there... take away the wider structure of society, and it's all gone. What specialists are left are scattered into countless different disparate enclaves, all with a little piece of a jigsaw puzzle (but many pieces lost beyond redemption post collapse) and no way to bring the pieces together to reassemble them.