I'm not in the power industry, but you can read more about the economics of retrofits here[1], and technologies here[2]. I'd imagine the cost of retrofitting depends on how much pollution needs to be mitigated to make the (pollution*dollars)/power figure acceptable.

This industry source[3] seems to have about as many stories about stations being decommissioned as it does about large retrofit projects, which says that in the US the cost assigned to the pollution often is more expensive than moving to another source.

Like a lot of environmental issues, I think concerns about emissions tend to be treated as though they are decadent compared to immediate economic impact, until a crisis is underway. That is, people notice their power bill, brownouts or their factory shutting down from a shortage of electricity, but they may not directly attribute their children's asthma or getting cancer a decade later to the power plant. Furthermore people can bemoan the waste of shuttering a functional power plant if the harm the decommissioning averted is not tangible to them. Crisis-level smog days can help justify policies that take action, as well as create an appreciation for the importance of air quality in general.

These jet engines sound like the equivalent of building a taller smokestack. They're probably a cheap way to lessen some of the health effects during a smog crisis, but not any sort of long term solution.

[1] https://www.scientificamerican.com/article/us-coal-fired-pow...

[2] http://nma.org/wp-content/uploads/2016/08/Fact-Sheet-Clean-C...

[3] http://www.power-eng.com/coal/coal-retrofits-upgrades.html

In some cases, I believe so. But a solar plant is only useful for half the day (at best).