Politicians interfering in where pensions are invested is a terrible idea. It won't stop at 5%, once the principle of politicians being able to direct where pension funds are invested is accepted they will not be able to resist the temptation to further "invest" this giant pool of money into pet projects.
The point of pensions is to encourage people to save to provide for themselves in older age, reducing the burden on the State. We shouldn't forget that, people have to have confidence that it's worth their while saving decades in advance of when they will need it.