This question was already answered by James R.
There are more examples of symbiosis, where it's better to cooperate than to dominate. E.g. lichen are made from algae and fungi. They can dwell in places where neither algae nor fungi could dwell on their own.
We have bacteriae in our intestines which help us to digest food. We eat for them, provide housing and shelter, they get a share in food even, but in return we get byproducts of their work which are beneficial for us and at least some of them help to keep bad bacteriae at bay.
It's better for us to keep them. Even keep them well and healthy, as new studies showed.
Trees and funghi also form symbiotic relations. Some plants have bacteriae dwelling in the roots, and it's a deal again - shelter and food supply in exchange to access to substances like nitrogene iones, which plants can't create on their own, but which they need for growing.
Domination isn't always the better choice.
All your examples of symbiosis came after the assumed mitochondria merger, not before. All your examples also allow both organisms to retain their integrity and autonomy. The bacteria that live in our gut have retained the genes that allow these to be autonomous. These bacteria have not lost most of their DNA, with the human gut doing that work form them.
The mitochondria have lost much of its DNA, with the cell needed to provide for the mitochondria. This is not the same as living in harmonious symbiosis. It is more like slavery, where the mitochondria became shackled, forced to work in the energy mines. Its legs have been broken so it can't function on its own, and leave like it came in. Its original autonomy has been gutted. This is not symbiosis.
The question of this topic is why do the mitochondria still have DNA? As an extrapolation of accepted theory, original cell and mitochondria, before the merger, would have been originally autonomous. The mitochondria will then enter the cell with hope of finding a friends that can share the work, in symbiosis. However, what it finds is a slave owner cell, who is lazy and need the modern mitochondria as a slave, since the host cell has lost its ability to take care of its own energy needs? To keep him working, did it need to shackle the mitochondria and even cuts off its legs, so it can't leave? The final mitochondria work force has two strong arms, but no legs, so it can work, but it can never leave; Hotel California.
Does this have mass appeal or is it easier to say these are not intelligent things with an agenda. Rather this the merger was all based on chemical equilibrium and not personification scenarios for those who reason with emotions induced by chance.
Does anyone have lab proof that some chance meeting and symbiosis is how the mitochondria and cell merged, or is this consensus science based on prestige; subjectivity? The consensus, has hundred in not thousands of labs, so I assume at least one of which should have done this in the lab by now.
My prediction is no proof will be offered, because none exists. We will just see insults to protect pseudo-science traditions, based on casino math and active imagination. There is no chemical logic offered. That is taboo.