Low pass filters can have significant phase delay, whereas Kalman filters can be made to have essentially no lag (phase delay). Additionally, Kalman filters can stabilize the estimate of a state based off of other correlated measurements much better than simply low pass filtering the measurement.
For example, in an IMU, the accelerometers may be noisy due to vibration from the aircraft or vehicle, and the magnetometer measurements which are not affected by vibration, can be used to stabilize the inclination estimates.
Kalman filters are extremely useful and enable applications not possible with just low pass filtering.
Also, you don't need to get the covariance matrix "exactly right", these are tuned in practice on actual measurement and can be used to speed up or slow down the state estimation or weighting of different measurements.
What makes you think fracking is not economically viable? The majority of the wells drilled in the US (by a large margin) are fracked. Fracking is very much an economical method for oil extraction right now, and is still viable at ~$50/barrel.
There’s even more opportunistic fracking when the price is higher, historically. A lot of wildcat fracking operations shut down when the price of oil drops.
While that's true, the era of wildcatter fracking being relevant to production in the US are largely over. The majors now entirely dominate the fracking segment and fracking-based oil production. They've gone on an acquisition spree across the past decade and tower over the smaller operators. All you have to do is look at the acres owned by the majors in fracking areas like the Permian vs small operators now (compared to the beginning of the fracking era).
Like this $9.5 billion deal the other day between Shell and Conoco. Wildcatters can't compete at these prices for the most part. They can't afford the real-estate now (in the early days when the industry was asleep and or skeptical they could).
The majors bring a more traditional, disciplined approach to pursuing cut-throat production competition or not and have a more consistent, less volatile schedule for capital investment. They're not as sensitive to medium size moves in oil prices when it comes to changing plans.
