True, but it is very popular in astrophotography.
Also, there are many recent advances in super-resolution imaging, compressive sensing, and sub-diffraction-limit pixels, often based on results from Donoho, Candes, and friends. Taking advantage of signal sparsity (i.e. that the set of physically possible signals is much smaller than the set of all possible signals, and physical systems have many constraints upon them) has led to a lot of surprising results over the past 15 years or so. Another approach that has yielded results is aperture synthesis, which has been applied to optical systems for at least the past 15 years. And in some certain circumstances for spysats, optical heterodyne detection can also be used to great effect.
> you are always going to be limited by the diffraction limit of the telescope
While the diffraction limit of a system is definitely a constraint, it's not as simple as this anymore.