There is a prevalent and also a very lethargic thought about the ending of Inception - "It does not matter if the final scene was real or not, all that matters is Cobb is happy and accepted what he saw as reality". I'm calling BS on this and am going to address the fact that there is nothing happy about that perspective; here's why.
Cobb and Mal were irresponsible parents!
Here's something of grave importance that nobody talks about. Cobb and Mal were parents of two lovely children. They knew well that they were experimenting with their minds, which could have dangerous side effects. Yet they placed their work ahead of their children, and both mommy and daddy simultaneously entered and experimented with deeper layers of their subconscious.
What was the outcome? Ding ding, no points for guessing! They lost their minds in Limbo and aged 50 years mentally thanks to the time dilation. Cobb literally had to perform inception of an idea into Mal's head to get her to believe her world wasn't real so that both of them could kill themselves to get back to reality. What comes off that? It results in Mal going crazy in the real world and killing herself, believing her world is fake. After that, Cobb had to flee the country to avoid arrest because Mal informed the authorities that Cobb was responsible for her death.
Not what one can call great parenting, yes? Now let's look at the situation from the two children's point of view.
From James' and Phillipa's viewpoint
Who are James and Phillipa? That's the name of their two kids. Let's see what went down for them. James and Phillipa see their parents return from work one day, and suddenly, their own mother is no longer looking them in the eye like she used to. This change happens overnight for the kids. They lose their mother's love instantaneously because Mal now believes James and Phillipa are mere projections in this fake world. Soon, the kids are told that their mother is no more. The next thing? Their father bolts, and they are left alone with their ageing grandfather. They wait for years, hoping that their father will return to them. But there is no saying if Cobb can successfully get back to them. If their grandfather dies, the kids will be orphaned and eventually separated. Doesn't look good, does it?
Now let's go back to the ending of Inception and address that theory - "It does not matter if the final scene was real or not, all that matters is Cobb was happy and accepted what he saw as reality".
The only way Inception's ending can be happy
It's pretty clear that Cobb and his happiness means squat compared to the fate of James and Phillipa. It's not sufficient that Cobb merely thought that we was back in reality, and all that matters is that he has made peace with what he's got because he's not looking at the spinning top anymore. No. There can be a happy ending only if the children are actually reunited with their father. A father who will now have realized never to mess around with his mind and dreams. From the perspective of the kids, who are the collateral damage of the experimentation of their careless parents, their ending pretty much sums up if it is happy or not. If that ending is in the real world, and Cobb is united with his kids, it's merely a second chance that life is giving him. But for the kids, if that last scene is not reality, they will grow up as orphans while their dad lies in some hospital bed with his mind happy lost in Limbo. Not joyous.
Thanks to that wobble we are shown on the top, physics tells us that the top is slowing down and that the last scene is a happy ending because the children have got back at least one of their parents.