I think this is an interesting line of thought.
How much money may Sony not-be-making by having their mount open to third parties?
We know Canon makes a lot of money with cheap cameras and lenses. They get most beginners with very attractive and affordable entry level cameras, then sell them a cheap 50mm, to attach them to the system. If beginners can't buy anything else, in a way, it is what it is - most likely, they'll commit.
Additionally, we see Canon is clearly trying to tackle the desire for third party manufacturers, with cheaper lenses that perform very well, like the 28-70 f/2.8, 16-28 f/2.8, and possibly this upcoming 45mm f/1.2 so then, a beginner that's looking to upgrade, will have attractive lenses available with the logo of the brand he trusts the most, which may help mitigate the desire for third party glass.
Sony is not that attractive to unknowing beginners, so a significant part of the line of though I just wrote will not apply - let's be real, a true beginner may not detach the kit lens for years. They have some outdated beginner cameras, and a few that are updated, but much more expensive, like the a6700. Also, their cheapest models are significantly more expensive than Canon's offerings (R100, R50).
At the moment, several of the most spectacular lenses for Sony FE are NOT made by Sony. Most of their users are not beginners either, for the reasons mentioned above, so they're not that afraid of buying third party - in fact, many go Sony because of the existence of such offerings.
There's the Tamron 35-150mm, Sigma's 28-45mm f/1.8, 28-105mm f/2.8, 35mm f/1.2 (two versions already!), 200mm f/2, 300-600mm f/4, and now the SongRAW 85mm f/1.2 and Laowa's 200mm f/2. There are probably a lot more awesome third party lenses I'm not aware of.
Sony has been repeatedly beaten on the race to groundbreaking lenses.
In an era where cameras are lasting longer and longer, due to slower improvements that can lead users to upgrade less often (hence camera brands pushing so hard on marketing for us to upgrade), and the use of electronic and, more recently, global shutters, if a brand with an open mount is not selling lenses, because there's so many decent third party options available, what will they sell then?