The cumulative number of exoplanets that have been discovered has been following power law scaling for the past 25 years and if this continued to 2050 then we would find about 100 million exoplanets.
20,000 exoplanets should be found from the past primary Kepler mission and the completed K2 mission and from the Transiting Exoplanet Survey Satellite and from the planned 2026 PLATO mission.
The Gaia mission should find between 15,000 and 90,000 depending on the final mission duration and the actual occurrence rates of massive planets in wide orbits.
The Wide-Field Infrared Survey Telescope is expected launch in 2025 and a mission duration of five years and it is supposed to find another about 70,000 to 150,000 transiting exoplanets, details depending mostly on the actual occurrence rate of hot Jupiters in the WFIRST observing fields. The total exoplanet count should be between 105,000 and 260,000 by 2030.
Rigidized polymers appear to be a feasible technology to create space telescopes larger than kilometer sizes. It should be possible to make 50-meter space telescope elements in 1000 kilometer baseline space telescope arrays.
The Event Horizon Telescope at 1.3 millimeter wavelength achieved 25 microarcsecond resolution on M87. Many 50-meter space telescopes in a 1000 kilometer baseline array would 100 times better with 245 nanoarcsecond resolution using 1 micron wavelengths.
50-meter space telescopes should be easily made from mylar inflation. Mylar inflation likely tops out at 100-meter space telescopes.
Larger monolithic inflated UV-cured solid apertures are theoretically possible up to 100 kilometers in size but we will need more space launch capabilities. They could be built after we have fully rapidly reusable launch capability.
Phased arrays of apertures are unlimited in aperture size.
Telescopes on the Earth and Moon can work together to create a 380,000 kilometer telescope array.
Once an OWL-type telescope is installed on the Moon, or even a 10-meter lunar precursor, one could readily address optical Intensity Interferometry with unprecedented baselines and angular resolution. For instance it could measure the heights of mountains on transiting exoplanets. This is an important problem for the geophysics of planets. Weisskopf (1975) has shown that there is a relationship between the maximum height of mountains on a planet and its mass and the mechanical characteristics of its crust.