I can only speak to particle physics, but the main issue is that we can't study the truly interesting problems. Theories such as string theory are basically bey

I can only speak to particle physics, but the main issue is that we can't study the truly interesting problems. Theories such as string theory are basically beyond the reach of experiments. Our current models work very well to describe the universe, but we haven't made that much serious fundamental progress since the 1970s. It takes decades to find new particles which we know must exist (top quark in 1995, Higgs in 2012). This will probably become even more true after the next few years after the LHC collects data at 13 TeV, although of course I could be wrong and something could be found. To study new physics you have to go up an order of magnitude in energy or luminosity, and this scales worse than linearly with cost, so it isn't feasible. Of course it is possible that new technologies emerge, but this isn't a sure thing. The other problem with physics is that it is really hard to become a professor, and the field forces 90% of bright, dedicated and talented people to go into industry because there is a lack of jobs in physics. We really need permanent positions at labs outside of academia.