Do you know (and are able to tell us) the reason for the change? I suppose i can understand it from a marketing point of view what with all the Space Marine stuff in the tabletop game labelled as "Space Marine". Though i would have also thought that going with a term that is solely/recognisably their own; "Astartes" (i don't know any other setting that uses that term) would be good to set them apart from every other setting that uses space marines (which is most sci fi).
I think that's part of the point. GW aren't trying to set themselves apart, they're trying to ensure if becomes a definitive, unified front. Such that when someone says 'space marine', you immediately think 'hulking, power armoured, genehanced warrior carrying the geneseed of the primarchs', not 'GW or that other setting?'
It's quite an aggressive stance to make, but also one which probably won't have verifiable/measurable results. A nice notion from the company's perspective.
There's also the undercurrent of 'Astartes' not being a word. That sounds like it's a big part of the motivation from the GW side too. "It is not correct to say 'Astartes', the phrase only makes sense when 'Adeptus Astartes' is said." Similarly, saying "Adeptus" on its own is probably a bit of an odd/over-general term. (Which has long bothered me with Ben's novel Dark Adeptus
, should've been Dark Mechanicus
Anyway, the 'silliness' of Space Marine as a phrase is diminished with use and conviction. Space Wolf sounds silly. Space Marine just sounds a bit odd. It's not the worst word out there though.
"The wooden thing in the living room?"
"That's the fireplace."
"Fireplace. It's the place where the fire goes."
"Top-notch naming skills, there."
Personally, I'm much more vexed by the appellation of 'brother-' to things. 'Brother-sergeant', for example. It seems ham-fisted. Too close to 'Wolfclaw' and 'Bloodmissile'. Even keeping the feel & unity pursued by using 'Space Marine', I think authors and creative wits can be a bit cannier in their approach to semiotics, innuendo & themes.
(McNeill, Abnett & Wraight did very well in their "There's Something About Magnus" trilogy.)