To be honest, I'd prefer more games did something similar to this. AAA titles are frickin expensive, and these days publishers seem to be more and more willing to cut costs where they can, understandably so in many cases.
Whether this is achieved by passing increased costs onto the player, adding costly DLC, skimping on story, lessening quality of gameplay, being lazy with level design or even just making what would have been a 12 hour game ten years ago into a 7 hour game today, somewhere along the line cuts are almost inevitable.
Except with graphics. When a sequel comes out, it better have improved graphics, cause if it doesn't look several shades better than its predecessor, you can bet it will come up as a sticking point in early reviews of the game.
And once, this made sense, if Metal Gear Solid 2 had come out with the same blocky animations and indeterminate facial expressions of Metal Gear Solid it would be understandable to have a go at Kojima for not improving his graphics.
But with something like Fallout 3, you're getting pretty damn near to as far as you can push it. Would you really prefer to have everything else that makes a game great suffer, just so the decaying flesh on Gob the Ghoul looks one iota more photorealistic than it did in Fallout 3?
No.
When a sequel comes out, it's meant to improve and build upon the previous release. If you could improve upon any aspect of Fallout 3, would it really be the graphics engine?
If a metric fuck-tonne of time, money and resources can be saved by using the same engine, and if said fuck-tonne of time, money and resources is invested into tweaking VATS, fixing up the skills and perks system and adding brand new characters, story, area, terrain, themes, motifs, enemies, factions, backstory and situation; then I am all for having it look like Fallout 3, safe in the knowledge that it doesn't need to look any different, because in everything else it will be new.
Of course, if they've just used the same engine and spent that saved time and money on booze and hookers instead, then I will be slightly less pleased.