No. If western countries let in 'everyone' who wants to come, they'd be horribly, horribly over-populated and huge swaths of the world would never develop.
The problem is (if you like game theory), that it's in everyone's personal interest to migrate somewhere richer. However, if everyone does that, wherever they came from experiences such massive drain that it'll be worse off. From a purely pragmatic standpoint, unlimited immigration is a poor 'equilibrium' for all involved. 'Developing' nations will never develop, and developed nations will face massive over-crowding.
The reason the west currently allows immigration in the numbers we do is because it's an easy way to get skilled, motivated labour - at once allowing for economic growth as well as driving down the relative cost of labour.
Freedom to travel/migrate should be a human right, however there are many problems with that, considering the current state of the world.
Yes. But clearing out your rubbish and learning what it's like to live with your not-rubbish is one good way to do that. This is one of the reasons she says you only need to go through the major tidying process once, ever.
One thing I like about the book is that she didn't write it until after years of experience helping people solve the way-too-much-junk problem, so it's based on what actually worked for a variety people who were failing to solve the problem on their own, unlike many self help books that are written only based on the author's experience of being naturally good at something.
It's important to put her focus on getting rid of stuff into context: Ms. Kondo is a "tidying consultant" whose clients are mostly affluent people that can't even get started on tidying up in any meaningful way because they simply have more stuff than storage. She tells stories of people throwing out 20, 30, 50 bulging full-size garbage bags of excess stuff - these are people with a lot of money and a lot of accumulated clothes and possessions, and who live in homes large enough to contain it but not organize it. They are also affluent enough to be able to err on the side of getting rid of too much stuff during an enthusiastic round of cleaning. Ms. Kondo exhorts her readers not to buy in bulk in order to avoid clutter - it's not like you need to be super-wealthy to pass up the cost benefits of buying in bulk, but her advice is not meant for people who are just scraping by or for whom the tradeoff of space-for-money makes sense.
I agree with your sentiment, but what do you do with all of the stuff you already have? Almost nobody reading these books is starting from a position of zero stuff; they're starting from a position of X stuff, with X being an amount higher than they'd like. The real appeal of these books and the philosophy they espouse is that they allow you to whittle down the number of things you already have whilst also learning healthy habits to avoid ending up in the same situation later.