I agree that in a low yield environment that investors will go towards riskier assets. However, it's kind of a large jump to go from gov/AAA bonds to watchs as an investment. Instead I think you just have people chasing yield through riskier BBB bonds as well as a greater confidence in the equity markets. Interestingly, I actually feel that watches as a whole are quite correlated with the stock market so if you're looking for diversification then I guess you're doing it wrong.
Instead, I think the run up in prices has been increased demand. Just more and more people going after fewer and fewer goods. I attribute a couple of factors to this:
1. Better journalism through blogs, forums, and especially Hodinkee. I know the Dink gets a bad rap sometimes here, but it was personally my gateway to realizing there was high quality content out there in an easy to read manner.
2. Better online resources/data to derisk buying a vintage speedy. Not only the good work that spacefruit has done with speedmaster101, but also the wide variety of resources this forum provides. Add to that the ability to quickly find various price points (watchrecon, ebay, chrono24) you have a larger audience who will feel more comfortable spending $7-8k for a used/vintage speedmaster than before when all you had was a blurry picture via email/craigslist and no data points to compare.
3. Social media. How I loathe and love it at the same time. Yes, Instagram (IG) has played a large part in showcasing the aesthetics of these watches and it has, in turn, developed a greater following which tends to continue to feed unto itself.
In short, I actually expect speedmaster prices to continue to appreciate, although not with as much velocity. As many have mentioned, condition will create a larger deviation in prices with better condition commanding a substantially larger premium. However, I do think we are getting close to the cap for non-straight lugged speedmasters.