Is there some idiom or trope to this that I'm missing that makes it obvious sarcasm? Because that's literally not possible without some inconceivable laws being passed, so I'm confused as to what you're trying to communicate.
Both got forked a bunch, but the mother project remains the popular one due to user stickiness and, frankly, sheer amount of (paid) effort invested compared to small-scale volunteer efforts. Users can still choose to go with Iceweasel, disabled Pocket integration or Ungoogled Chromium or whatever fork de jour, but generally they don't because "the ecosystem keeps moving". They generally get more desirable things added than annoyances. Unless a company gets it wrong and you get a WhatsApp-level backlash to make another competitor viable.
With constantly moving goalposts, commercial interests can keep the upper hand as the community (and much other competition) can't keep up fast enough. That's why Google keeps pushing the web into ever new directions, and why standardized APIs got replaced by ever-changing Electron apps with mandatory auto-update.
If the popular distributions ship with ads, I'm not afraid as long as there is competition out there that does without. I can always do Arch or Debian instead of Ubuntu and I know they'll do the right thing.
Where it gets dangerous is when the base technology is controlled by commercial interests. Microsoft owning secure boot certificates, Android not getting drivers upstreamed, Apple deciding who gets to compete by owning the App Store. Linux may end up in this spot at some point if a particular vendor (or consortium of commercial interests) becomes too powerful compared to the community and smaller competition. Once it gets there, the only escape is having viable alternatives.
If you rely on commercial software, your alternatives come with huge switching costs. Chromium to Firefox is a little tedious, Mac to Linux is a major disruption. The more you rely on a particular piece of commercial software, the more they can control you.
But competition generally does the trick. I don't mind games being closed source, there are so many quality alternatives that the game studios simply can't lock most users in. Now, app stores for games? That's where you want to be cautious, because they control what competition looks like. Operating systems and drivers? If they turn on you, better start swearing and leaving investments behind. Chip-level DRM? Zero leverage, ask your government for help and if they're not willing, you're screwed.
So really, commercial software isn't that much of an issue high up in the stack. The lower you get, the more careful you want to be about end user communities retaining control. Let the companies make apps, let them sell apps, don't let them grow platforms. The platforms should belong to users.
If a company comes along and makes a commercial platform out of Linux, run.
Once your default distribution starts shipping with built in ads, and every other distribution follows suit because it makes them boatloads of money, your linux desktop will be well and truly dead.