FB doesn't claim selling the company is the only reason to start one.
But it is a top 3 "exit strategy" for startups, so limiting it would have a chilling effect on startup formation. How big that effect would be is probably a better avenue for counter arguments.
I'd imagine that anyone interested in buying Giphy would fall into one of two categories:
1. Large tech companies for which the same antitrust arguments would apply.
2. Companies that can't afford to pay anywhere near $400M for a GIF sharing service.
What makes a free GIF sharing service worth $400M dollars - apart from the data harvesting Facebook can do to better target users with ads.
We don't need to outlaw these sales, just outlaw the integration of data from purchased services, and then you'll see what the true value of the company is.
Both of which fall under arguments 1 and 2
Giphy is free to sell itself to almost any other company on Earth except FB. This has no chilling effect whatsoever
I'm okay with that. Startup culture is toxic, and the world would be a much better place if the only legal reason to found a business would be to make a steady profit selling your product. If your company only exists so you can sell it (as in the company as a whole) to someone else, your company shouldn't exist.