I dont know how often things change on a MX apart from a plastics facelift, new engine ? or a new tune on a old one ? Probably still a standard steel cage frame, air system, brakes ? Tyre technology ? Steel handlebars that have not changed much in 20yrs... Im sure the suspension keeps getting better but their gears don't grow by one more every year or so as they have a gearbox and standard chain, standard wheels ect...
These days, a 1 year old MTB doesn't have all 'new' stuff, the stuff that apparently makes it 1% stiffer and 0.1 of a second faster... yes some get caught up in it, yes many people have said to me 'Strewth, you could buy one with an engine in it for that price'.
There's a price on small gains and new technology that help something that is 100% human powered to go faster or easier, whereas a few kilos on an e-bike or 5-10kg on a dirtbike doesn't really matter.