Sounds like public domain. What’s the difference between the two then?
Public domain, other than by copyright expiration or (in the US) federal government creation is actually not clearly possible to create in a legally unambiguous way in many regimes with automatic copyright and no direct provision for unilateral transfer to the public domain; the most permissive open source licenses approximate it while providing better legal clarity.
Open source licenses often seek to protect the originator from liability they might otherwise incur from chain-of-commerce relationships, making it safer for them to make the software available without fee, and to prevent misattribution of origin. Other open source licenses, while not having field of use limits, seek to assure that derivatives are also offered on similarly-free terms. There's quite a variety, within the bounds of certain essential guarantees that are central to the common value proposition.
Some people feel just as strongly about imposing field-of-use restrictions to exclude military entities as you do about excluding "trillion dollar companies". Others feel strongly about excluding all commerical usage.
But "Open Source" means no field-of-use restrictions — so although any of you are free to craft your own licenses excluding X, Y, or Z, you don't get to call those licenses "Open Source".
> I fear we're letting perfect be the enemy of the good here, and letting the FOSS ecosystem wither and lose incalculable value by letting trillion-dollar corporations exploit the hard work of people for free.
No economic value is lost. Timescale is free to publish their own software offings with their own licensing; those offerings merely exist outside the Open Source ecosystem.
Open Source is plenty successful. It does not need to destroy itself in order to encompass all things.
Besides, "letting trillion-dollar corporations exploit the hard work of people for free" is what Open Source has always done. Everybody gets to use it for free — trillion dollar corporations, governments, religious organizations, activists, convicts... everybody!