I haven't really been able to fully figure the loyalty system either. I know standard of living is a main component of it, but I haven't found a good way to reliable keep loyalists around. This is what it says on the wiki, but it doesn't really help with figuring out what to do:

Radicals and loyalists, just like with interest group membership, are not whole pops but rather individuals within pops. Loyalists can be considered happy or content, while Radicals are unhappy.


There is a large variety of ways that pops can become radicals or loyalists, here's a few of the more common reasons listed below: 

   Pops that experience an increase in SoL will become more loyal.

   Pops from political movements that have their demands fulfilled become more loyal.

   Pops that experience a decrease in SoL will become more radical.

   Pops from political movements whose demands are ignored may radicalize over time.

   Pops whose SoL is below the minimum they expect to have will radicalize over time, particularly if it's so low that they're actually starving.

   Pops that are discriminated against (ethnicity/religion) radicalize over time (but only if they are literate!)

...

Pops will remain radical or loyalist until they either die or have a status change as a result of becoming more radical/loyal (for example, a loyalist pop might stop being loyalist if their material standard of living suddenly takes a nosedive), but they do, in fact, die.
As generations die off and are replaced by new ones, less and less people will remember all the great things done for the country 30 years ago and will start wondering instead what has been done for them lately.