Fair enough, however, it still doesn't explain why somebody buying a motherboard today would NEED to have a board with PCIe 4.0 if they upgrade every four years. In fact, it makes even LESS sense, because in four years there probably WILL be some benefit to having PCIe 4.0 because the device, OS and driver support will likely have caught up by then, at which time they can get a motherboard that is PCIe 4.0, or by then, likely 5.0 if indications are accurate, since they will be upgrading at that time anyhow based on your belief in a 4 year upgrade cycle.
Doesn't make sense to buy a board today, with features that won't benefit you for four years, if you are going to buy another board in four years, if you can buy one today without it that saves you money. Now, if there are OTHER features you need on a particular chipset, then it makes sense, or if you "just want it" then who is to say you shouldn't? Just saying, it also makes a lot of sense for a majority of people to NOT choose to go with a more expensive board when these B450 boards will already offer everything else for significantly less money.