You can do a lot with the PC and the right products, even with older neglected paint.
I have never used anything more abrasive than PB SSR 2.5 or Meg's #83 DACP and the paint has to be fairly bad to need those.
In most cases, SSR2 or Meg's #80 will do the job. I used to use Meg's #9, but I have gotten away from it.
On all the vehicles I have worked on, I could go right to a LSP from any of those. From what I have read, that is not always the case. If you have some haze left after the DACP or the SSR2.5, then that is where you might need another step.
If I am using a sealant, I follow the swirl remover product with Klasse AIO and then apply the sealant. If it is on my own vehicles, I would follow the swirl remover with Klasse AIO prior to a wax as well.
If I'm doing a freebie for a friend, I use Meg's #80 followed by Meg's NXT Tech Wax and then send the bottle of NXT home with them. (They pay for it)
As has been mentioned, keeping the finish up is a lot easier than restoring it. I usually use a swirl remover 1 or 2 times a year, Klasse AIO about every 3 to 4 months and recently 4 Star UPP or Collinite IW about every 4 to 6 weeks.
It probably is more than necessary, but it is my hobby and doesn't have to make any sense.
Charles