One of Japans biggest issues is also one of population. It's declining... And will decline by 24% by 2050 with current projections, that has a big impact on economic capability.
So if Japans economy stays stagnate whilst it's population declines, then per-capita economic input should in theory be increasing.

I guess being strict on immigration isn't always a good thing.

All of those entities encountered issues such as either demographics, geography, geopolitics, and poor policy or a combination thereof that prevented them from eclipsing the US aside from the current China which doesn't seem to have many of the issues that effect those geopolitical entities to the same degree so far ... 

Japan was arguably never going to be a real contender for reasons such as poor geography which meant little land and natural resources so their aging demographics manifesting only made it much clearer. India had massive issues with poor policies the past. Russia or it's predecessor didn't care about innovating which made them fall very behind American technology and as the old soviet joke goes "they pretend to pay us, we pretend to work" made the industrialized working class realize that they were paying for sub-par goods/services so Soviet Russia as a whole was forced to take a bitter pill known as free market capitalism or choose to rot in their dark ages. The European Union has geopolitical issues which means their so called "solidarity" amounts to nothing in the face of nationalism where Southern Europeans nations disapprove of a common currency and the recently admitted Eastern European nations are refusing political interference from the EU as much as possible ...

Be careful for what you wish for, immigration has the potential to change the geopolitical make up of nation. It's a likely possibility that if Japan does open up to immigration because of declining native demographics, it'll likely be Chinese constituents that'll migrate because of their still less than ideal conditions from their nation of origin ... 

What do you think would happen to Japan if it had to face a much bigger perceived threat than North Korean nationalism like Chinese nationalism where it's current representative constituents could become a minority in only a several generations ? What do we do if Chinese nationalists does become the clear majority of Japan's representation and they vote to get annexed by a still authoritarian state just like we saw with Crimea/Russia ? 

Just as how former North Korean or unified Korean kingdom constituents facing discrimination is well documented in Japan, if Chinese nationalists do gain political prominence on a massive scale how can you be so sure that it won't devolve into a civil war like we saw with confederate/union America ? 

It's easy to suggest immigration as a solution to Japan's declining demographics but I doubt many have went on to fathom farther enough of the potential consequences and I don't think it's in American or other liberal western democracy's interests to have Chinese influence in their biggest ally in Asia ... 

If the EU was a unified stable state it would be the world's superpower.

Hopefully someday we will be. 

Unfortunately, there are exit movments in many countries :(

Famous last words ;)

So you don't believe in science? 

I do, I do believe in the real track record of data captured on earth, and so far, the actual data differs a lot from the computer models IPCC has been using. 

Evidence to backup your hypothesis?

I won't post evry single source I follow, cause I am tired of posting too many links.

This video is a good start.