It sure has helped the rest of the world conquer poverty. That doesn’t seem like stagnation to me.
The more we have immigrants stocking supermarket shelves and constructing infrastructure the more we can focus our high end R&D on things like biotech, medicine, space, and the like. That’s economics; comparative advantage is real.
That's not what OP is arguing. He appears to be arguing that technological solutions should not be tried in cases where immigration can solve the problem.
If Japan can automate many of these things they will end up being leagues ahead of those who solved it with immigration and will end up selling the solutions to the rest of the world.
But why should they focus on solving "problems immigration can solve" as opposed to "problems immigration cannot solve?" You could end up leagues ahead of the rest of the world in something like biotech or materials sciences. There's no intrinsic gain to be had by weighting the problem space.
The more we have immigrants stocking supermarket shelves and constructing infrastructure the more we can focus our high end R&D on things like biotech, medicine, space, and the like. That’s economics; comparative advantage is real.