Casey,
I can't promise they would last forever. I am a salesman, not an engineer. The proper person to tell you the lifespan of something like this would be a PE (Professional Engineer). However I can address a few things.
1) Titanium is the most cathodic metal commonly used for structual purposes (the only metals I know higher on the chart are gold, silver, and platinum). Though carbon fiber masts are actually more cathodic than any metal, as far as I know.
2) Like steel, titanium has an infinite fatigue limit, which means so long as cycle loads are kept below a % of breaking strength cycle loads will never effect the part. The limit for 316 stainless is around 30%, for grade 5 titanium it is around 50%
3) titanium has roughly 2.5 times the tensile strength of 316 stainless.
4) Titanium weighs about 1/2 of 316
5) Titanium is considered immune to salt/brackish/polluted water corrosion
6) Titanium can be subject to crevice corrosion at tempratures in excess of 200 degrees F
7) Titanium is much more abrasion resistant than 316 (sand laden water pipes can carry about 3 times the flow rate if using titanium vs steel tubing)
Generally titanium is just a better metal that 316 for marine use. Of course the primary brake on adoption by the marine market place has been it higher costs (which are justified to some extent), and concerns about machining (which are not).
Figure a non-standard part, like a single chain plate it will run about 3 times the price of stainless, but will last much longer, are stronger, and lighter. If you are looking to have multiples made (say all your chainplates at once) then I can probably get a volume discount that would reduce cost (likely in the twice the price range). I can't promise the prices though until I run them through our quoting system. If you could get a whole fleet of boats to purchase at once (say an owners group of the same boat) the price would come down again.
The best price I have seen comparing titanium to stainless was for a couple of our most commonly used bolts, where the price was about 20% more than stainless, but to get this low we have to make a lot of something.
Edit - After retreading this, I wanted to note that while I do sell titanium, all of this information is strait out of engineering texts, not advertising publications. If you would like to confirm any of the above, we use primarily Grade 5 titanium for marine uses due to its strength, and is the alloy I was referencing in the above. A number of different alloys are commercially available, and they do have different physical properties.