SotN is another example, if Richter dies the castle is destroyed. It wouldn't be easier to Shaft let Richter die and still ressurect Dracula? The problem is that he simply can't since Richter was the actual Castlevania's master.
No, the castle which Richter is the master of is technically
not Castlevania. Alucard/ Richter discuss this after Richter is saved and it's mentioned Drac is in "that castle over there". The 2 castles are inversions of one another and connected by the throne room, that's it. If the warp portals(keyholes) transported Alucard between the 2 castles, then this would be a different story, but they don't, the 2 castles are separate entities.
That's exactly what shaft wanted. It would have been easier for Richter to die and for shaft to resurrect Dracula. However, shaft couldn't destroy Richter, therefore the best he could do was to control him. He may have also known Alucard would rise (maybe not?) but he didn't expect Richter to be saved, nor would he want him to. The fact that you have to find the holy goggles to save Richter infers this.
As well as the fact Shaft says that you're "too late", Dracula's resurrection has begun. Shaft would have much preferred that Richter died, this would have made Dracula's resurrection much easier for him (for example Richter dying during the events of Rondo/ Rondo).
I also believe that if Shaft wasn't controlling Richter, Alucard would have been killed by him. I'm not saying Alucard is weak, I'm saying he's weaker than Richter, who has already destroyed Dracula once.