a lot of the changes in markers have come about from changes in rules and changes to what is required.
when everyone shot semi auto everyone wanted a gun that could shoot as fast as possible, nothing else mattered, guns weighed twice what they do now, had bits bolted on to them and most of them were a royal pain in the ass to keep them working consistently, if it shot 30 bps it didnt matter if it only shot 4 pods from a tank, it didnt matter how heavy it was or how tall it was, no one cared if it needed a full strip clean and service after every other case of paint, its job was to throw as much paint as possible and not break the paint. Most serious players had at least 2 guns and in most cases they cost more than a top end gun does these days
then fixed rates of fire came in and markers slowly became more and more refined, right now the average player looking to buy a top end marker wants something that shoot nice and smooth and doesn't kick (because they are all soft and never played when guns didnt kick because they weighed a tonne), is light, easy to maintain etc...
the technology in the guns will continue to advance as long as there are advancements to make and as long as changes to requirements occur, gone are they days when you spend £700-800 on a gun then at least the same again to customize it and bring it up to standard, guns these days work out of the box to do the job they are designed to do.