Comment by tzs

That's not how it works. First-to-file (FTF) did not change the requirements for patentability. You still have to have invented the thing you want to patent, and in your scenario Big Corp did not invent the thing.

All FTF changes is what happens when multiple inventors invent the same thing.

Under first-to-invent (FTI) your priority date was the date you conceived the invention if you then worked diligently toward reducing the idea to practice up until you filed your patent application. If you stopped working diligently on reducing the idea to practice and then resumed it, the date you resumed became your new priority date.

What counts as a break in working toward reduction to practice sufficient to reset your priority date? How much documentation do you need to prove you were working continuously on it from your claimed priority date?

Figuring all that out can be expensive and time consuming and often gives results that seem wrong. It's almost random whether the priority date by this method actually matches who seems to morally most deserve the patent.

FTF gives priority to whoever files first. It doesn't produce any worse outcome than FTI and saves a lot of time and money for both the patent office and applicants.