I think perhaps more telling than GW no longer showing the regular old marines in promotional images is the fact that they haven't made any new kits/refreshed old kids for the regular marines in years.
The most recent mini-marine models made for regular retail channels to my memory were the Deathwatch Veterans (which came out ~2 years ago or more, and are a marginally larger scale than the old Tactical Marines). Armageddon or Kill Team both could have been opportunities to come out with new models, but they got either the old Scout models (which are really starting to show their age) or Reivers.
For non-standard retail, there were the formerly-Japan-exclusive Space Marine Heroes. But those feel more like a novelty/collector's item than an actual made-for-gaming release.
Eventually those old Scout, Terminator, Devastator, and Tactical Marine molds are going to wear out. When that happens -- in several years still, but eventually -- I doubt we'll see them replaced. Classic marines will either be a novelty for old gamers (much like metal models are now). Rules-wise, they'll probably stick around for the lifetime of the molds, but after that I can see some bit of fluff saying that the last of the Space Marines has finally crossed the Rubicon Primaris, and the old Marines will be written out of the rulebooks completely.
In the nearer-term, as the number of Primaris kits increases, less and less shelf space is likely going to be dedicated to the old kits. Command Squad, Devastator Squad, Vanguard & Sternguard Squads, and the like will probably move out of regular stock and into Online Only status. It's just a question of physical space in stores; you need more room for new stuff, and if that new stuff effectively supplants old stuff, the old stuff is what gets moved.