2009 MacBook Air: I can, and want to, purchase an SSD upgrade, but what about Mavericks > Sierra? I have a 2009 MacBook Air with Mavericks. (It’s old, but low miles.) I was about to pull the trigger on buying a SSD to improve performance, but I just now learned that I won’t be able to upgrade to Sierra (?). Only 2010 and later models for MacBook Air are supported according to apple.com.

https://www.apple.com/macos/how-to-upgrade/

I’m floored. Am I really stuck with Mavericks forever? Should I just forget about upgrading the hard drive, since I cannot install Sierra anyway? Is the pitiful 2 GB of RAM (which cannot be upgraded ☹️) unable to handle any more OS’s? 

What can I do, then, with this perfectly ok, if painfully slow, machine? Has anybody done this against recommendation and had it turn out ok?

FWIW, my primary use for this machine is going to be development work, i.e., self-study, writing &amp; testing scripts in the terminal, etc. I don’t do much gaming, video, or heavy graphics, etc.
Replaicing your rather slow spinning HD with an SSD will certainly make your Mac faster. In fact its the one upgrade that will give you a noticeable speed bump. Other things can give you some speed like RAM (not an option for you) but the SSD is a great upgrade.

That said Apple does not support upgrading to Sierra (though there are hacks that will allow you to do so, which is beyond the scope of this site.)

The only way to see if it is to slow for your intended usage is to try it and see. Personally, 2gb of RAM is considered pretty small these days and I would be hesitant to upgrade the OS because of that. But depending on your use case it might be acceptible with the addition of an SSD which should speed things up considerably.