I'm really not sure about this one at all. For example, let's look at my 'flavour of the week'....Juniors. Who would have thought that 50+ years after being sold for $49 that some would be reaching $7000. Now someone putting 49 bucks in the bank in 1955 and went every couple of day to the bank and asked to look at it, and play around with it for a while, would be sitting on a little more than 49 bucks today. However, would it be as much as 7000....I don't do sums like that, but I'm thinking the cash would lose out on the junior. Also you would have more fun playing and looking at the guitar then you would taking the twice weekly walk to the bank.
Investments are never easy, well good ones aren't. you would have to pay your money and take your chance. Stating 10 years is a little unrealistic for an average guitar, but 25+ might show a better pay back. So 25 years of joy (and sometimes pain) of guitar playing could be seen as priceless.
Buy what you want, play it when you want and wait to see what happens.