> What is another telescope good for?

My understanding is that WFIRST is more of a replacement for Hubble as it slowly dies (or suddenly dies at the end of a fiscal year due to $$$). However compared to both JWST and Hubble, it has a much higher field of view (the example that NASA gave at one point is that took the Hubble several years to survey the Andromeda Galaxy in IR. WFIRST could do that same job in a few hours). This has a lot of benefits if you want to do things like look for exoplanets. WFIRST will of course be very expensive, but for a number of reasons it is unlikely that it will be even close to the cost of JWST (like JWST it will be going to the L2 Lagrange point, but it won't have to make as many design compromises as JWST to get there)

The last decadal survey of astronomers[1] determined that WFIRST was a #1 priority because:

"...WFIRST will settle fundamental questions about the nature of dark energy, the discovery of which was one of the greatest achievements of U.S. telescopes in recent years. It will employ three distinct techniques—measurements of weak gravitational lensing, supernova distances, and baryon acoustic oscillations— to determine the effect of dark energy on the evolution of the universe. An equally important outcome will be to open up a new frontier of exoplanet studies by monitoring a large sample of stars in the central bulge of the Milky Way for changes in brightness due to microlensing by intervening solar systems. This census, combined with that made by the Kepler mission, will determine how common Earth-like planets are over a wide range of orbital parameters. It will also, in guest investigator mode, survey our galaxy and other nearby galaxies to answer key questions about their formation and structure, and the data it obtains will provide fundamental constraints on how galaxies grow. The telescope exploits the important work done by the joint DOE/NASA design team... and expands its scientific reach. WFIRST is based on mature technologies with technical risk that is medium low and has medium cost and schedule risk... WFIRST will complement the targeted infrared observations of the James Webb Space Telescope. The small field of view of JWST would render it incapable of carrying out the prime WFIRST program of dark energy and exoplanet studies, even if it were used exclusively for this task. The recommended schedule has a launch data of 2020 with a 5-year baseline mission. An extended 10-year mission could improve the statistical results and further broaden the science program. The European Space Agency (ESA) is considering an M-class proposal, called Euclid, with related goals. Collaboration on a combined mission with the United States playing a leading role should be considered so long as the committee’s recommended science program is preserved and overall cost savings result."

>How many astronomers and support staff do we need in this industry?

I'm going to be honest, this comes across as a bit of a loaded question. Obviously we don't "need" any astronomers at all (or most scientists for that matter). But more theoretical research can still have tremendous payoffs. I'm a big fan a letter[2] that a paperclip'd director at NASA Marshall wrote to a nun in Zambia who asked why the US spends billions going to the moon while children on Earth starve to death.

>How do we measure and know that it's enough?

I don't think humanity will reach a point in any scientific endeavor where we decide that it's enough and we don't need to understand any more. I suppose it would be when we have a complete understanding of the entire universe.

[1]: https://science.nasa.gov/astrophysics/special-events/astro20...

[2]: https://lettersofnote.com/2012/08/06/why-explore-space/