Is a Lofstrom loop (also known as launch loop) realistic?
The general idea behind a Lofstrom loop is a large structure intended for easier access to space, held up by active support (think rotating chain)
Yes or no, depending how you define realistic.
Like many other fantastical future technologies like space elevators, cylinder habitats, and nuclear lightbulb drives, there's nothing wrong with the underlying concepts. Physicists have written papers, done the math, and found that it mathematically could work. There are no known laws of physics that prevent space elevators from being built or launch loops from existing. So, from a physics standpoint, a lofstrom loop is totally possible.
From a practical point however, it gets dicey. Lots of these megastructures, such as the lofstrom loop, rely on active support technology or materials science that's a bit better than what we have now. These are things that have never been demonstrated on a large scale. Additionally, while possible, the engineering would be a nightmare and the sheer scale is something that very few if any modern engineering projects can match. For example, the loop requires an iron rotor to be moving through a sheath at 14000 m/s which is just insane. It can also never stop, as then the structure would collapse, and the amount of stored energy in the belt rivals that of nuclear weapons. Also, the mechanism to turn the rotor around would be under immense forces.
In summary, I'll speculate a bit: We won't see a lofstrom loop being built in the next 50 years, if perhaps ever. The problems inherent with actively supported structures are just too big to easily overlook, especially if compared to "simpler" space launch megastructures (or even fully reusable rockets). If building one ever becomes possible, other, simpler technologies will probably be far more mature and offer cheaper bulk orbital-lift capabilities.