People shouldn't shy away from posts like this; it is a necessary discourse to have concerning what is acceptable of a AAA company these days.
Fallout 3 was released 7 years ago, and New Vegas was developed by Obsidian Entertainment, so Bethesda Softworks has had 7 years, 4 from Skyrim, to draft a new game and obtain the rights to a better engine, make a new one, or overhaul the DX9-utilizing dinosaur they've been using since Morrowind— since 2002.
This isn't about shiny graphics, because people generally understand that graphics aren't everything *cough*Bloodborne*cough*, this is about determining if it is acceptable for a large development team to create new products with dated material. This isn't about how the game looks, exactly, or Bethesda alone, but the laziness of the industry/studio(s). What nodding your head in approval or defending their actions does is tell Bethesda, and by extent other game developers, 'yeah, we'll except outdated engines and mechanics inelastically,' not pushing the industry to improve and bring up-to-date, better content. The answer is simply no, it is inexcusable to make a new game with the same engine and framework that was designed for Windows XP in the year 2015/16.
And you can't say that 'modders can just fill the gaps' because that's enabling them to be lazy. People shouldn't rely on someone else not connected to a company to provide the ultimate service that the company states it does. I'd love to start a project and then have other people finish it for me for free and then I get the money from sales because they made it what it should have been. If they aren't going to put full effort into it then they shouldn't be allowed to charge full price.
Even if we can't force Fallout 4 to receive the upgrade(s) it needs, we can still let it be known that this kind of below-the-bar work isn't welcomed for future games.