I think Canon will negotiate license agreements that stipulate what lenses Sigma and Tamron can make - allowing the 3rd party companies to make lenses that Canon has no interest in making. The agreement that Nikon has with Tamron is rumored to be such an agreement - and, of course, I have no way of knowing if this is true. If such an agreement can be made, Canon profits with far less negative impact on their own lens sales.
Sifting through various Facebook groups, there is no doubt that the news of "Bad" Canon not allowing 3rd party lens makers to make RF lenses has been bad publicity for them (usually making this a blanket statement and not mentioning it is only AF lenses, and making it sound like Canon has officially declared that this a policy that will definitely not change in the future). There are people posting, who for this very reason, have bought their first - or next camera - from another brand. So far, the numbers are no doubt small, and it has not hurt Canon's position in the market, but bad publicity, once it gets on social media, can not be a good thing - especially when it is exaggerated or not completely accurate. Just look at Sony - there first two generations of mirrorless FF cameras were very sub-par (in my opinion as a 40 year photographer), but social media told us how great they were and they sold well and established Sony as the number 2 brand. So best not to have a bad reputation on social media in today's world, so it seems.