How to change the system font?

Apple has provided no solution for this yet and appears not to be concerned. This font and size was clearly created to work best on Retina / 5K at the detriment of all the other existing displays. Force obsolesce? Migration incentive? I don't know - but I do know it's far from ideal and this cannot be a simple mistake.

Does Apple think everyone is endowed with 20/20 vision?!

I just completed the update, and once again Apple has blundered in its fixed fonts - or more to the point not allowing users to set their own system wide font size and contrast level.

People complained that the system wide "font is too small and there is no way to enlarge" issue back during the transition to OS X. I thought Apple would have addressed this in an update as sweeping and important (visually speaking) as Yosemite as it represented a major departure from Aqua and skeuomorphism. This has apparently fallen on deaf ears in Cupertino.

This is pretty unacceptable. My Windows system allows me to scale the system fonts. I am on a 15" non-Retina MBP and have good vision (mild case of nearsightedness; only need to wear glasses sometimes when trying to read a white board from the back of the lecture hall).

My glasses case often has a thin layer of dust on it, I wear them so infrequently.

Well, that won't be for long. I'm sitting maybe 24" from my screen and after this update, I keep finding myself squinting to read the typeface in the URL bar, browser tabs, and bookmarks. For instance, Safari, the tab names on inactive tabs is not only laser cut thin the contrast is poor. Thin dark charcoal letters on a light charcoal background; who thought this was good for legibility?

I haven't fully combed through the O/S yet but I presume this problem carries on throughout Yosemite. I bought my parents a MacMini this summer and jumped on the phone an hour ago to tell them not to update! Their vision is considerably poorer and I know they would struggle with this version of OS X even though I have it hooked up to a 24" HD display.

Apple released this as a pre-test Beta and I cannot believe people, especially those in the Art & Design community, didn't speak up about this issue. Helvetic Neue or not - I could care less. I'm largely ambivalent about the font style. I just want the ability to be able to enlarge the the font and contrast so I can read without strain and pain.

I hope there's a 3rd party fix or update soon.

* Apple if you're reading this - c'mon already! You learned in iOS7 that a number of people had trouble with the thin, fixed fonts and low contrast and offered a more scalable, thicker font option with a higher contrast color (deeper blue) option in one of the 7.X updates. Is it asking much to have the same options and flexibility in OS X?