OK, I've had time to kill, so I'm going to prove that instruments are bad investments, even the vintage ones.
Currently on ebay there is a 1959 Fender P and they are asking $13,500. Let's assume they get it.
I don't know the original sticker price of a P in 1959, but for argument's sake let's say it was $300. Adjusted for inflation, that's just over $1600 in today's dollars. So your "profit" is around $12,000, right?
Now, take that same $300 dollars in 1959 and invest it in something that earned, say, an index fund or something that would have earned a conservative 8% over the same period of time (I'm using historical trends, not specifics here). Now, that $300 would be $22,000. Your opportunity cost, what Baird referred to, is $10,000, or nearly twice what your profit was from the Fender. That's what investments do. They (ideally) grow and make money over time. While a vintage instrument could technically be considered and investment, it's really only if you bought it 40 or 50 years ago. And even then, it's a pretty marginal one. If you bought that 59 P today, you'd be lucky to keep up with inflation. You'll never "make" money off it.
I'm no financial pro, so feel free to correct. But overall I think this is right.
Buy the instrument you want and can afford. Don't worry about financial loss- it's inevitable.