Purchasing guitars as investments is much like purchasing physical precious metals. I'll use my uncle as an example here because he actually does all of those things. He has about 70 guitars in his collection, all considered "collectible". However, his profit on these instruments has probably broken only slightly north of even. Apart from his passion for instruments, he wanted them as security. But he buys them with profits from higher risk/higher yeild investments and pockets the difference.
Like the aforementioned investments, instruments are useful as wealth maintenance not for their growth potential. And, like precious metals, there is a risk you lose money, but you can't lose it all. You can do it poorly (IE buy a bushel of squiers for 500 a pop, or pay 3000 an ounce for gold) but if you have a lot of a given item, you will likely maintain your wealth and possibly grow it. With guitars/guns/cars etc, a large collection helps to ensure that when some loose market value, others gain and you end up about even. If you want to purchase a single item as an investment, it needs to be a pretty guaranteed investment quality purchase (1st Gen Colt SAA, Shelby Cobra 427, vintage Gibson ES-335) which will cost you EXACTLY what it is worth at the time. Your return at best will be 1:1 the day you buy it. With most guitars/cars/guns whatever, it's less than that the minute you put your money down.
Any given thing may become collectable, but the thing about physical items is you never know. I buy stuff and keep it as long as is viable for me, and sometimes I sell for profit, sometimes I just get rid of stuff. The nice thing is I have all kinds of crap I can unload if I need some money.