For-profit companies have a legal duty to their shareholders to prioritize short-term growth and profit over all else. While I have gripes with Sesame Workshop as an organization (see: affiliation with Autism $peaks and SeaWorld, anti-union activities, participation in advertising campaigns, various blunderous creative decisions, etc.) they are not beholden to this like any for-profit would be.
This means their sole focus can (and should) be on creating educational, informative, and research-backed childrens content for underserved communities.
It might not be immediately clear why this would be an inherently bad thing, so let's walk through a potential alt-timeline in which Sesame Street sold out.
Firstly, the wealth of free content Sesame Workshop provides would be one of the earliest components to feel this impact. Cut would be the free ebooks, games, videos, and interactive web content Sesame Workshop produces every year, with this content either being paywalled or deemed simply too expensive to keep making. Further, it wouldn't be long until Sesame Street ended its (financially limiting) affiliations with longtime partner PBS, likely moving to a cable television or streaming exclusive model. This means the show would be inaccessible to the impoverished youth that the show was originally designed for.
A lesser discussed aspect of Sesame Streets production is its heavy involvement with child development researchers and educators who work together with writing staff to create season long lesson plans revolving around the up to date needs of its young audience that are based in scientific research. This is also an aspect of Sesame Streets production that costs an enormous amount of money, and therefore would likely be swiftly scaled back or gutted completely. Because of the way for profit companies operate with an impossible goal of infinite growth, when you reach a point of market saturation (wherein everyone knows about and owns your product already) the only way to keep up the exponential growth that is legally demanded is to start cutting costs.
This leads us to the main point, the quality of the show itself would inevitably suffer. While we've seen some of this in the past decade and a half as a result of ballooning production costs in the transition to HD and the defundung of PBS forcing Sesame Workshop to find outside financing, it would be much, much worse here. Any aspect of the show that does not translate to immediate short term profits would inevitably be gutted and replaced with trend-chasing and toy-hocking advertainment. If you take a look at Sesame Streets contemporaries, you might get a better idea at what I mean by this. Shows like Mickey Mouse Clubhouse, Blaze and the Monster Machines, or Paw Patrol use their superficial educational components as a jumping off point to sell Cable TV packages, Toys, Theme Park Tickets, and in the last case, state sanctioned violence. Has Sesame dabbled with these ideas? Sure, who among us didn't grow up with a Tickle Me Elmo, but it has never been the shows main focus.
Lastly, do you know what's *really* expensive? Fair compensation for labor. UNIMA, Puppeteers of America, and SAG-AFTRA all fight to ensure the Muppet Performers are fairly compensated for their work and kept safe on the job. A for profit company (as Disney had done for many years in its parks) would likely see these expenses, along with the continued decline in the popularity of puppetry among young children, as unjustifiable expenses. You know, computer animation is really hip with the kids, and as its an un-unionized industry produced largely over seas, they can pay people in poor working conditions pennies to produce it.
I hope this helps illuminate why this would be a bad move, I know its a large text wall but such is the nature of complex issues like these.