Ultimately all computers get dropped from OSX upgrades. When a model falls behind on given specifications Apple will not support it in an OSX release, or it may make the upgrade but certain features will not be supported in older models. So typically a higher model computer such as one along a similar line of models but with slightly higher CPU might make one final cut whereas others get dropped. It depends upon where the line gets drawn and what features Apple may be introducing and their hardware requirements. We can't predict what Apple will do but we can look at what they have done in the past. Feature support starts tailing off when a computer is 4-6 years old but a computer may not lose support completely until it is 8 years old. However, things have changed. I still run a 14 year old G4 but gone are the days when that could be done because you just can't do hardware upgrades the way you used to.
My late 2008 MacBook is the last of the MacBook line to meet El Capitan specs. and not many 2007 models of anything make it. It is 2016 so 2016-2008 = 8 years of OS upgrades. It would not surprise me at all if my computer never sees another OSX upgrade. By the way, El Capitan really likes 8 GB RAM which is the maximum I could install in this computer so perhaps one way of looking at it is saying you only get the maximum OSX life if you also have the maximum RAM possible for a model.
I can't say which would be best. When I did grad. school there was one PC8088 for use by 20 grad students. We were overjoyed when they installed a 10 MB hard drive. My thesis fit on a 360k 5" floppy. It depends upon what kind of computing your wife needs to do and what her patience level is. Remember, I keep computers for 13 years, nor do I own an iPhone, iPad, iPod, or iTunes Store account. 🙂. I personally view MacBook Airs as being for those who need something small and light and few features. Not for the person who needs external drives or wants to be hard wired to the Internet, but good for the business person who needs ultimate portability and can afford a new computer every 5 years unless they don't mind not having the latest and greatest. The old MacBook Pro stands the best chance of being a DIY upgrade computer but is also going to be the first one to need those things. It is likely also heavier. If I were getting one I might be tempted by it but I tinker with computers; I have a high patience level with old things (my 100 year old house and 35 year old car). My wife has less patience with such things and went with a newer model MacBook Pro a few years ago and decided on a cost per year basis it met her needs.
Get 16 GB RAM to try to future-proof. More would be better but might be expensive unless you are really thinking long-term.
