its different for every manufacturer.

90s machines basically had a cpu running code from eeproms which just read switch matrix states / drives lamp matrix outputs etc... usually all the boards in the backbox.

newer machines swapped to faster microcontrollers running the show with node boards under the play field driving all the hardware linked by some sort of serial bus.

latest machines are basically embedded pcs/socs running linux (far easier to drive screens etc...) but every company does it slightly differently.

If you want to play best bet is something like Proc or Fast controller.

Swapping to a tiny controller and all the drivers on a serial bus was a pretty big change from the older systems. I don't think the boot time is an issue definitely wont effect machines on location and maybe slightly annoy machines in peoples homes. SMD boards is far far cheaper for the manufactures to design / assemble in volume with an added bonus (like you say) no one can repair them so you have to just buy another one from them if they fail (win win)

I personally think its all going in the right direction and is definitely progress on the older systems using the best technology available. If you want these feature/video rich machines you need the powerful software/hardware to create them which is a os driven system as its far easier to create video content on a os with a file system / drivers etc... then on a micro-controller.

indeed that is what i was saying its win win for the manufacturer, cheap boards + extra income when they break.

Home pin has kept with through hole for this very reason.

I'm not so sure, depending on component size some of it is workable but the components are getting so small now you got no chance unless you have very specialised equipment i think 98% of people would not have the tools or knowledge to repair this stuff