Without Jane, The Reality Stone Would've Never Been Found
Wherever Bor had hidden the Aether turned out to be an ideal hiding place, remaining truly lost since long before Thor, Loki, or Heimdall were born. Until Jane found it. Jane Foster may have washed her hands of her brush with the Aether - by which we mean having an Infinity Stone select her as a host - and who could blame her? But the fact remains, if she hadn't gone searching for "readings" that seem more meaningless than ever now, the Reality Stone may have stayed hidden. Almost certainly would have, since even now we can't say where it was in the universe.
The Reality Stone was ultimately given to The Collector to protect, which Thor believes a stroke of genius, and Star-Lord finds the stupidest possible decision. By the time the Guardians arrive on Knowhere, Thanos has already claimed it from The Collector, so Peter Quill seems to have the stronger case.
And as powerful as the Reality Stone is on its own, had it remained hidden, Thanos never could have wiped out half the universe.
That's due to the fact that the Infinity Stones are far more powerful when joined together. They demonstrate a range of powers on their own, of more or less potency or service to the plot. Without the Reality Stone, Thanos couldn't have gotten the drop on Gamora and the Guardians... couldn't have removed Drax and Mantis from the fight... couldn't have turned Star-Lord's gunfire into bubbles.
As some would argue, it's still possible that Thanos would have turned to other Stones to achieve the same goal (part of the reason why the game-breaking powers of the Stones can make storytelling difficult). But in the end, it takes all six to achieve Thanos's mass culling across the universe. Without the power over reality, it couldn't have been achieved.
Unfortunately, this placement of blame is impossible to state with absolute certainty, since it's not actually clear how Thanos deduced the Stones' locations. Through spying and surveillance, as he did in the comics? Or by simply 'tracking' them (the less interesting option)? If Jane hadn't tripped over the Reality Stone, there would be nobody to even interrogate for the location.
No Jane, no Reality Stone. No Reality Stone, no completed Gauntlet. No Gauntlet... no snap. Natalie Portman may be open to an MCU return, but frankly, she's done enough already.