I don't for a minute think GW will phase old marines out of the tabletop, some models will go to legends for sure.
Consider that GW has been more profitable than ever and frequent space marine releases have been part of that. They did not drop the primaris line all it once but have trickled it out through all of 8th. I expect frequent space marine releases is going to be a standard practice for them moving forward, they will never want to go an extended period of time without having new stuff for space marine players to buy.
That being said, eventually it will make less and less sense to keep adding to the primaris line, what then? Old marines 2.0, in the same scale as CSM 2.0. This is a profit gold mine and there is no way GW will ignore it.