Considering building and selling spare parts is higher profit than assembling new tractors, not keeping up on part availability over time would be a very bad business practice (unless it is some low volume model with many unique parts). If you priced the parts to build your own I think you'd find buying the parts alone would pretty much equal or exceed the cost of buying one assembled and you don't have the costs of factory assembly and testing associated. Selling parts is where most of the money is to be made for manufacturers.
I exist at the butt-end of a manufacturing line, and am associated with several others of several various other industries, and I can say without a doubt most manufacturers make the majority of their money on their product after the sale. The manufacturing divisions make the most money on spare parts and in the product services end of things is repairs. Many assembly lines bleed money which is why companies are always looking for automation and cheap labor, without parts and repairs, manufacturers and the dealer networks would disappear quicker than they could be setup... Those same companies seeking automation and cheap labor also dole out millions on figuring a way to provide more services and improve their parts networks. The focus of where manufacturers are putting their money versus where they try to save their money is a pretty good indicator of where their money is made...
As for how long parts will be available? Who knows... Joe Farmer back in 1945 I'm sure wasn't thinking "will parts be available for my grandkids to do a full ground up resto on this Massey in 2013?"... What I do know is that the more of a product they can sell, the longer they can sell parts and services for it. The more of something they can sell and the longer they can sell parts and services for it, the higher the profit margin is on that particular product. I don't see goals of shorter lifespans on the "new iron" being beneficial to anyone, especially manufacturers. I think as long as the manufacturer is around, there will (at a minimum) be parts available for their most popular models. If only 1000 of (name of most popular old iron ever built goes here) had ever been built, parts wouldn't be available today.
I'm not saying this holds true for all the tractors produced either. The world is producing more and more "disposable products" all the time, and I'm sure there are more than one "disposable" tractors being sold that the manufacturer's have no intentions of supporting past the warranties. But that doesn't mean all new machines will suffer that same fate, it just doesn't make good business sense...
I think the primary reason to go new over old or vice-versa, would be how well you know and can service a given tractor. An old tractor is great as long as you can work on it. If you need to buy a tool to work with, and you don't know how to fix what may break; you are likely best off with a new machine that you can learn about over time doing routine maintenance. Someone who knows that old Ford and can look at it and see what is out of place, it's likely a great $2500 investment. Someone who's never spent any time with one and has no clue what would be out of place or not could likely find themselves paying triple that $2500 investment in parts and labor bills because they weren't familiar enough with the machine. In my case, I personally don't want major repairs at this point in my life. I've got hundreds of hours of seat time on other people's machines and never saw under the hoods of any of them. I picked something I thought would last for my uses for what time I have left on this side of the pasture. If something happens to it, hopefully it will either be under warranty, or I'll have learned enough about it through general maintenance and reading in my spare time that I can handle whatever it is. I have work to do and don't want to be learning how to fix something on my tractor to get the work done I need done if it can be avoided in any way... What happens to it once I'm pushing up daisies in the pasture is up to my boys and part availability...
